Immunoglobulin libraries

ABSTRACT

Methods and compositions for the screening and isolation of ligand-binding polypeptides, such as antibodies. In some aspects, methods of the invention enable the isolation of intact soluble antibodies comprising a constant domain. Screening methods that employ genetic packages such as bacteria and bacteriophages enable high through-put identification of ligand binding molecules.

CROSS REFERENCE TO PRIOR APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 60/867,936, filed on Nov. 30, 2006, which isincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of proteinengineering. More particularly, it concerns improved methods andcompositions for the screening of combinatorial antibody librariesexpressed in bacteria.

2. Description of Related Art

Currently recombinant therapeutic antibodies have sales of well over $10bn/yr and with a forecast of annual growth rate of 20.9% they areprojected to increase to $25 bn/yr by 2010. Monoclonal antibodies (mAbs)now comprise the majority of recombinant proteins currently in theclinic, with more than 150 products in studies sponsored by companieslocated worldwide (Pavlou and Belsey, 2005). In terms of therapeuticfocus, the mAb market is heavily focused on oncology and arthritis,immune and inflammatory disorders, and products within these therapeuticareas are set to continue to be the key growth drivers over the forecastperiod. As a group, genetically engineered mAbs generally have higherprobability of FDA approval success than small-molecule drugs. At least50 biotechnology companies and all the major pharmaceutical companieshave active antibody discovery programs in place.

The original method for isolation and production of mAbs was firstreported at 1975 by Milstein and kohler (Kohler and Milstein, 1975), andit involved the fusion of mouse lymphocyte and myeloma cells, yieldingmouse hybridomas. Therapeutic murine mAbs entered clinical study in theearly 1980s; however, problems with lack of efficacy and rapid clearancedue to patients' production of human anti-mouse antibodies (HAMA) becameapparent. These issues, as well as the time and cost consuming relatedto the technology became driving forces for the evolution of mAbproduction technology. Polymerase Chain Reaction (PCR) facilitated thecloning of monoclonal antibodies genes directly from lymphocytes ofimmunized animals and the expression of combinatorial library offragments antibodies in bacteria (Orlandi et al., 1989). Later librarieswere created entirely by in vitro cloning techniques using naïve geneswith rearranged complementarity determining region 3 (CDR3) (Griffithsand Duncan, 1998; Hoogenboom et al., 1998). As a result, the isolationof antibody fragments with the desired specificity was no longerdependent on the immunogenicity of the corresponding antigen. Moreover,the range of antigen specificities in synthetic combinatorial librarieswas greater than that found in a panel of hybridomas generated from animmunized mouse. These advantages have facilitated the development ofantibody fragments to a number of unique antigens including smallmolecular compounds (haptens) (Hoogenboom and Winter, 1992), molecularcomplexes (Chames et al., 2000), unstable compounds (Kjaer et al., 1998)and cell surface proteins (Desai et al., 1998).

During the past decade several display methods and other libraryscreening techniques have been developed for isolating antigen specificbinders from large ensembles of recombinant antibody fragments. Thesetechnologies are now widely exploited to engineer antibody fragmentswith high affinity and specificity. Many of these screening platformsshare four key steps with the procedure for antibody generation in thein vivo immune system: first, the generation of genotypic diversity;second, the coupling of genotype to phenotype; third, the application ofselective pressure; and fourth, amplification.

Phage display is currently the most widespread method for the displayand selection of large collections of antibody fragments (Hoogenboom,2002). In this methodology, a gene of interest is fused in-frame tophage genes encoding surface-exposed proteins, most commonly pIII. Thegene fusions are translated into chimeric proteins in which the twodomains fold independently. Phage displaying a protein with bindingaffinity for a ligand can be readily enriched by selective adsorptiononto immobilized ligand, a process known as “panning”. The bound phageis desorbed from the surface, usually by acid elution, and amplifiedthrough infection of E. coli cells. Usually, 4-6 rounds of panning andamplification are sufficient to select for phage displaying specificpolypeptides, even from a very large libraries with diversities up to10¹⁰. The most successful application of phage display include thefollowing: first, the de novo isolation of high-affinity human antibodyfragments from nonimmune and synthetic libraries (Griffiths et al.,1994; Vaughan et al., 1996; de Haard et al., 1999; Knappik et al., 2000;Hoet et al., 2005), including antibody fragments against self antigens;second, the generation of picomolar affinity antibodies by in vitroaffinity maturation (Yang et al., 1995; Schier et al., 1996; Lu et al.,2003) and third, the discovery of antibody fragments with uniqueproperties from non-immune (Chames et al., 2000; Huie et al., 2001) andimmune libraries from animal or human donors (Moulard et al., 2002;Kramer et al., 2005).

Ribosome and mRNA display represent another method for the display andscreening of libraries of antibody fragments. The concept relays on thestable formation of a complex of antibody fragment and its encoding mRNA(Lipovsek and Pluckthun, 2004). In ribosome display, the link betweenantibody fragment and encoding mRNA is made by the ribosome, which atthe end of translating this mRNA is made to stop without releasing thepolypeptide. The ternary complex as a whole is used for the selection.In mRNA display, there is a covalent bond between the antibody fragmentprotein and the mRNA which is established via puromycin as an adaptormolecule. These display methods are carried out entirely in vitro.

In microbial cell display screening is carried out by flow cytometry. Inparticular, Anchored Periplasmic Expression (APEx) is based on anchoringthe antibody fragment on the periplasmic face of the inner membrane ofE. coli followed by disruption of the outer membrane, incubation withfluorescently labeled antigen and sorting of the spheroplasts. APEx wasused for the affinity maturation of antibody fragments (Harvey et al.,2004; Harvey et al., 2006). In one study over 200-fold affinityimprovement was obtained after only two rounds of screening.

Nonetheless, all high throughput antibody screening technologiesavailable to-date rely on microbial expression of antibody fragments.The use of antibody fragments rather than intact or full length IgGs, inthe construction and screening of libraries has been dictated bylimitations related to the expression of the much larger IgGs inmicroorganisms. IgG libraries have never before been expressed orscreened using microorganisms such as bacteria or yeasts. As a resultthe isolation of antigen binding proteins has been carried outexclusively using antibody fragments that are smaller and much easier toproduce. Once isolated such antibody fragments have to then fused tovectors that express full length immunoglobulins which in turn areexpressed preferentially in mammalian cells such as CHO cells.

Antibody fragments, including Fabs and especially single chain Fv's(scFv), pose several limitations: (1) Antibody fragments often exhibitlow affinity for the target antigen. Unlike IgG or IgM antibodiesantibody fragments are monovalent and therefore they cannot capitalizeon avidity effects for stronger binding to antigens (Pini and Bracci,2000). Thus, the isolation of recombinant antibody fragments to targetsthat cannot be recognized with high affinity e.g. carbohydrates, isproblematic. (2) Antibody fragments generally exhibit lowerthermodynamic stability than their corresponding full length IgGcounterparts (Worn and Pluckthun, 2001). (3) Because of their small sizeand their lack of an Fc region, antibody fragments exhibit very shortcirculation half-lives compared to full length IgG proteins (Milenic etal., 1991). Therefore for the vast majority of clinical applicationsantibody fragments isolated from combinatorial libraries have to beconverted to full length IgG molecules.

The need for isolating intact IgG molecules from combinatorial librarieshas long been recognized. In an attempt to establish a platform for theisolation of recombinant IgGs, researchers recently displayed smalllibraries of IgGs on the surface of mammalian cells. After homologousintegration of a single-gene copy in each cell, the population wassorted by flow cytometry to obtain single selected clones (W. D. Shen,Amgen, cited by Hoogenboom (2005)). Nevertheless, this technology istime-consuming, cumbersome, and expensive and is not amenable to thescreening of large libraries comprising of many tens of millions ofdifferent antibody proteins. The present invention overcomes theselimitations by avoiding the need for expression of IgG libraries inmammalian cells. Instead, the inventors have devised methodologies forthe screening of libraries produced and secreted by E. coli bacteria.

E. coli possesses a reducing cytoplasm that is unsuitable for thefolding of proteins with disulfide bonds which accumulate in an unfoldedor incorrectly folded state (Baneyx and Mujacic, 2004). In contrast tothe cytoplasm, the periplasm of E. coli is maintained in an oxidizedstate that allows the formation of protein disulfide bonds. Notably,periplasmic expression has been employed successfully for the expressionof antibody fragments such as Fvs, scFvs, Fabs or F(ab′)2s (Kipriyanovand Little, 1999). These fragments can be made relatively quickly inlarge quantities with the retention of antigen binding activity.However, because antibody fragments lack the Fc domain, they do not bindthe FcRn receptor and are cleared quickly; thus, they are onlyoccasionally suitable as therapeutic proteins (Knight et al., 1995).Until recently, full-length antibodies could only be expressed in E.coli as insoluble aggregates and then refolded in vitro (Boss et al.,1984; Cabilly et al., 1984). Clearly this approach is not amenable tothe high throughput screening of antibody libraries since, with thecurrent technology it is not possible to refold millions or tens ofmillions of antibodies individually. The expression of full length IgGantibodies in secreted form in E. coli was reported only recently(Simmons et al., 2002). However, there is no information on whetherantibody libraries consisting of many different antibodies can also beexpressed in bacteria. Equally importantly, the prior art does notdisclose any methods for isolating E. coli cells expressing a particularIgG with specificity towards a desired antigen from a vast excess ofcells expressing IgGs of unrelated specificities.

E. coli expressed antibodies are not glycosylated, and fail to bind tocomplement factor 1q (C1q) or FcγRI and thus cannot elicit complementactivation or mediate the recruitment of macrophages. However,aglycosylated Fc domains can bind to the neonatal Fc receptorefficiently (FcRn). Consequently bacterially expressed aglycosylatedantibodies exhibit serum persistence and pharmacokinetics similar tothose of fully glycosylated IgGs produced in human cells.

SUMMARY OF THE INVENTION

In a first embodiment the invention provides a Gram negative bacterialcell wherein the cell comprises (a) an antibody-binding domain expressedin the periplasm of the Gram negative bacterial cell and (b) an antibodybound to said antibody-binding domain. The skilled artisan willrecognize that a Gram negative bacteria comprises an inner and outermembrane and a periplasmic space between the two membranes. Thus, anantibody-binding domain of the invention will be anchored on or in theinner membrane and may be positioned such that antibody-binding domainis exposed to the periplasmic space. Thus, an antibody bound to such anantibody-binding domain will be expressed in the periplasmic space.Methods for expressing polypeptides and in particular antibodies in theperiplasmic space are known in the art for example see U.S. Pat. No.7,094,571 and U.S. Patent Publ. 20030180937 and 20030219870 eachincorporated herein by reference. In some cases a gram negativebacterial cell of the invention may be defined as an E. coli cell.Furthermore, in some preferred aspects a Gram negative bacterial cell ofthe invention may defined as a genetically engineered bacterial cellsuch as a JUDE-1 strain of E. coli. Gram negative bacterial cells of theinvention may be viable or non-viable cells. Furthermore, in someaspects, Gram negative bacterial cells of the invention are defined ascells lacking an intact outer membrane such that the periplasmic spaceis in contact with the extra cellular environment, in some cases cellslacking an outer membrane are referred to as spheroplasts. Methods fordisrupting, permeablizing or removing the outer membrane of bacteria arewell known in the art for example see U.S. Pat. No. 7,094,571. Forinstance, the outer membrane of the bacterial cell may be treated withhyperosmotic conditions, physical stress, lysozyme, EDTA, a digestiveenzyme, a chemical that disrupts the outer membrane, or by infecting thebacterium with a phage or a combination of the foregoing methods. Thus,in some cases, the outer membrane may be disrupted by lysozyme and EDTAtreatment.

In further aspects of the invention, an antibody bound to anantibody-binding domain that is comprised in the bacterial periplasm maybe anchored to the inner membrane of a bacterial cell. Such an antibodybinding domain may comprise a constant domain (Fc) such as animmunoglobulin heavy chain constant domain. Thus, in some aspects of theinvention an antibody may comprise two immunoglobulin heavy chains andtwo immunoglobulin light chains. In certain cases, each of the heavy andlight chains comprise both a variable and a constant domain or fragmentsthereof. However, in some aspects, immunoglobulin heavy and light chainsof the invention comprise functional domain fragments of the variableand/or constant domains. As used herein the term functional domainfragment means that antibodies composed of said fragments are capable ofbinding to a target antigen (in the case of a variable domain fragment)or an Fc specific antibody-binding domain (in the case of a constantdomain fragment). Furthermore, immunoglobulin chains comprising afunctional domain fragment may be defined by their ability toefficiently assemble into an intact antibody that comprises 2 heavy and2 light immunoglobulin chains. Thus, in some specific embodiments, anantibody of the invention may be defined as an intact antibody or ineven more specific cases an antibody may be defined as a full lengthantibody. Furthermore, in some aspects, an antibody for use in theinvention may comprise an antibody heavy chain but no light chainpolypeptide. Thus, an antibody may comprise VH, CH1 and CH2 domains. Instill further aspect an antibody for use in the invention may be definedas comprising VL and Ck or CL domains.

A skilled artisan will recognize that an antibody for use in theinvention may be an IgA, IgM, IgE or IgG antibody. Preferably andantibody of the invention is an IgG antibody such as an IgG1, IgG2a,IgG2b, IgG3 or IgG4 antibody. Furthermore, the antibody may be definedas a human or humanized antibody. Such an antibody comprises amino acidsequences found in endogenous human antibodies.

In some further aspects, a Gram negative bacterial cell of the inventionfurther comprises a nucleic acid sequence encoding an antibody. Theencoded antibody may be any of the antibodies defined herein. Thus, itwill be understood that a nucleic acid sequence may encode animmunoglobulin (Ig) light chain, an Ig heavy chain or both. In somecases an Ig light chain and heavy chain may be encoded on separatenucleic acid molecules and may be expressed under the control ofhomologous or heterologous promoters. However, in some preferredaspects, the Ig light and heavy chains are encoded on the same nucleicacid molecule. For example, an Ig light and heavy chain may be expressedunder the control of a single promoter in a discistronic expressioncassette. In further aspects a nucleic acid of the invention comprisessequences that facilitate Ig export into the periplasmic space. Suchsequences are well known in the art and may comprise a secretion signalfused to the Ig chain (U.S. Patent Publ. 20030180937 and 20030219870).Furthermore, an antibody encoding nucleic acid may comprise additionalelements such as a phage packaging signal, an origin of replication or aselectable marker gene. In some preferred aspects the Ig encodingsequences are flanked by known sequences such that the Ig sequence maybe amplified by PCR using primers that anneal to the known sequence. Insome very specific cases a nucleic acid encoding an antibody may be thepMAZ 360-IgG vector such the one shown in FIG. 2.

In a further aspect of the invention a Gram negative bacterial cell isfurther defined as a population of bacterial cells wherein cells in thepopulation comprise (a) an antibody-binding domain in the periplasm ofthe Gram negative bacterial cell and (b) a plurality of distinctantibodies bound to said antibody-binding domain. Furthermore, apopulation of bacterial cells may comprise a plurality or a library ofnucleic acid molecules encoding distinct antibodies. As used herein a“distinct antibody” may be defined as an antibody that differs formanother antibody by as little as one amino acid. While, in some casesdistinct antibodies may comprise differences in the sequence of a lightor heavy chain constant region, in preferred aspects distinct antibodiescomprise amino acid differences in a heavy chain variable domain, alight chain variable domain, both the light and heavy chains or adifferent combination of light chain and heavy chain. Methods for makinga library of distinct antibodies or nucleic acids that encode antibodiesare well known in the art and exemplified herein. In certain cases, aplurality of antibodies may comprise antibodies with a plurality oflight and/or heavy chain variable domains cloned from a population ofB-cells or B-cell precursors. However, in other aspects, a plurality ofdistinct antibodies may comprise light and/or heavy chain variabledomains with essentially random amino acid sequences. Thus, in someaspects, a population of gram negative bacteria according to theinvention may be defined as comprising at least about 1×10³, 1×10⁴,1×10⁵, 1×10⁶, 1×10⁷, 1×10⁸, or more distinct antibodies. In some cases apopulation of Gram negative bacterial cells may be produced by a methodcomprising the steps of: (a) preparing a plurality of nucleic acidsequences encoding a plurality of distinct antibodies; and (b)transforming a population of Gram negative bacteria with said nucleicacids wherein the Gram negative bacteria comprise an antibody-bindingdomain anchored to bacterial inner membrane and wherein the antibodiesare expressed in the periplasm.

A variety of antibody-binding domains are known in the art and may beused in the methods and compositions of the invention. In some aspects,an antibody-binding domain binds to the constant (Fc) domain of anantibody. It will be understood that such an antibody-binding domain mayhave specificity for a particular type or subtype of Ig, such as IgA,IgM, IgE or IgG (e.g., IgG1, IgG2a, IgG2b, IgG3 or IgG4). Thus, in somepreferred cases the antibody-binding domain may be defined as an IgGbinding domain. Some antibody-binding domains for use in the inventioninclude but are not limited to a binding domain from one of thepolypeptides of Table 1. For example, an antibody-binding domain may bean antibody-binding domain encoded by a FCGR2A, FCGR2B, FCGR2c, FCGR3A,FCGR3B, FCGR1A, Fcgr1, FCGR2, FCGR2, Fcgr2, Fcgr2, FCGR3, FCGR3, Fcgr3,FCGR3, Fcgr3, FCGRT, mrp4, spa or spg gene. Furthermore, anantibody-binding domain may be defined as a mammalian, bacterial orsynthetic binding domain. For instance, a bacterial antibody-bindingdomain may be a S. aureus protein A or protein G domain. In certainaspects a synthetic antibody-binding domain may be the ZZ polypeptide.

In some embodiments of the invention an antibody-binding domain isanchored to the inner membrane of a Gram negative bacteria. Methods andcompositions for the anchoring of polypeptides to the inner membrane ofGram negative bacterial have previously been described (U.S. Pat. No.7,094,571 and U.S. Patent Publ. 20050260736). Thus, in some aspects, anantibody-binding domain may be fused to a polypeptide that is associatedwith or integrated in a bacterial inner membrane. Such a fusion proteinmay comprise an N terminal or C terminal fusion with an antibody-bindingdomain and in some cases may comprise additional linker amino acidsbetween the membrane anchoring polypeptide and the antibody-bindingdomain. In certain specific cases, a membrane anchoring polypeptide maybe the first six amino acids encoded by the E. coli NlpA gene, one ormore transmembrane □-helices from an E. coli inner membrane protein, agene III protein of filamentous phage or a fragment thereof, or an innermembrane lipoprotein or fragment thereof. Thus, a membrane anchoringpolypeptide may be an inner membrane lipoprotein or fragment thereofsuch as from AraH, MglC, MalF, MalG, MalC, MalD, RbsC, RbsC, ArtM, ArtQ,GlnP, ProW, HisM, HisQ, LivH, LivM, LivA, LivE, DppB, DppC, OppB, AmiC,AmiD, BtuC, ThuD, FecC, FecD, FecR, FepD, NikB, NikC, CysT, CysW, UgpA,UgpE, PstA, PstC, PotB, PotC, PotH, Pod, ModB, NosY, PhnM, LacY, SecY,TolC, Dsb, B, DsbD, TouB, TatC, CheY, TraB, ExbD, ExbB or Aas. In someparticular aspects of the invention, a membrane anchoredantibody-binding domain may be a NlpA-ZZ fusion protein such as the oneexemplified in SEQ ID NO:4.

Furthermore, in certain embodiments, the invention provides a nucleicacid sequence encoding a fusion protein wherein the fusion proteincomprises an inner membrane anchoring polypeptide and anantibody-binding domain. Such nucleic acid vector may comprise a numberof additional sequences including, but not limited to, an origin ofreplication, a promoter, a phage packaging signal or a selectable markergene. In some very particular embodiments a nucleic acid sequence of theinvention may encode a NlpA-ZZ fusion protein as exemplified by thepBAD33-NlpA-ZZ vector.

In a further embodiment of the invention, there is provided a method forisolating a genetic package comprising a nucleic acid encoding aligand-binding polypeptide comprising (i) obtaining a population ofgenetic packages comprising a plurality of nucleic acid sequencesencoding distinct ligand-binding polypeptides wherein said packagescomprise the distinct ligand-binding polypeptides in complex with asecond polypeptide that binds to the ligand-binding polypeptides, (iii)contacting said population with a target ligand and (iv) selecting atleast one genetic package based on binding of a distinct ligand-bindingpolypeptide to the ligand. In some cases, the ligand-binding polypeptideis defined as an antibody or fragment thereof (e.g., as describedherein). Thus, in certain aspects, the second polypeptide that binds tothe ligand-binding polypeptide may be defined as an antibody-bindingpolypeptide or a polypeptide comprising an antibody-binding domain.

As used herein the term genetic package refers to a structure thatenables association between a ligand-binding polypeptide and a nucleicacid encoding the ligand-binding polypeptide. Thus, in some embodiments,a genetic package may be a bacterial cell or a bacteriophage as furtherdescribed supra. For example, in cases wherein the genetic package is abacterial cell, such as a Gram negative bacterial cell, the distinctligand-binding polypeptide may be expressed in the periplasm of the Gramnegative bacterial cell. For example, a population of Gram negativebacterial cells may be produced by a method comprising (a) preparing aplurality of nucleic acid sequences encoding a plurality of distinctligand-binding polypeptides and (b) transforming a population of Gramnegative bacteria with said nucleic acid sequences wherein the Gramnegative bacterial cells comprise the ligand-binding polypeptides in theperiplasm along with a second polypeptide that binds to theligand-binding polypeptides. In some further aspects, the secondpolypeptide that binds to the ligand-binding polypeptide may be anchoredto the inner membrane of the bacterial cell thereby tethering theligand-binding polypeptide to the bacterial inner membrane. Methods foranchoring a polypeptide to or in a bacterial inner membrane are known inthe art and further described herein. Thus, in certain aspects, a methodof the invention may further comprise (ii) disrupting the outer membraneof a bacterial cell(s) prior to or concomitantly with contacting thebacterial cell(s) with a target ligand.

Methods for selecting a genetic package based on binding (or lackthereof) to a target ligand are further described herein in the contextof bacterial cells or bacteriophages, however such methods areapplicable to selection of any genetic package. For example, a ligandused for selection may be immobilized or labeled to facilitate theselection process. Selection according to the invention may comprise twoor more rounds of selection wherein the sub-population of geneticpackages obtained in a first round of selection is subjected to at leasta second round of selection based on the binding of the distinctligand-binding polypeptide to the target ligand. Furthermore, in somecases a selection method of the invention is further defined as a methodfor producing a nucleic acid sequence encoding a ligand-bindingpolypeptide, further comprising (v) cloning a nucleic acid sequenceencoding the ligand-binding polypeptide from a selected genetic packageto produce a nucleic acid sequence encoding a ligand-binding polypeptidewith a specific affinity for a target ligand. Furthermore, a nucleicacid cloned from a genetic package may be expressed in a cell to producea ligand-binding polypeptide. Thus, in some aspects, methods of theinvention may be defined as methods for producing a ligand-bindingpolypeptide with a specific binding affinity for a target ligand.

In yet a further embodiment there is provided a method of producing abacterial cell comprising an antibody having specific affinity for atarget ligand. The foregoing method may comprise (i) obtaining apopulation of Gram negative bacterial cells comprising (a) anantibody-binding domain in the periplasm of the Gram negative bacterialcell and (b) a plurality of distinct antibodies bound to saidantibody-binding domain as described supra. Next the antibody is (iii)contacted with a target ligand under conditions wherein the targetligand contacts the antibody, and (iv) the bacterial cell is selectedbased on the binding of the candidate antibody to target ligand toproduce a bacterial cell(s). In some specific aspects a method of theinvention may additionally comprise the step of exposing the antibody tothe extra cellular environment by (ii) disrupting the outer membrane ofthe bacterial cell (e.g., forming a sphereoplast). Thus, in certaincases the antibody binding domain may be anchored to the inner membraneof the bacterial cell as described supra. The skilled artisan willreadily understand that in some cases the bacterial cell produced may beamplified by allowing the isolated cell or cell population to propagateunder permissible conditions. Furthermore, a bacterial cell produced bythe methods of the invention is included as part of the instantinvention. Preferably, Gram negative bacterial cells for use in methodsof the invention comprise an expression vector encoding a plurality ofdistinct antibodies. Such cells may be non-viable, but in a preferredembodiment the cells are viable and thus capable of being grownfollowing production/isolation.

The skilled artisan will understand that methods for selecting cellsbased upon their interaction (binding) with a ligand are well known inthe art. For example, a ligand may be immobilized on a column or bead(e.g., a magnetic bead) and the bacterial cell binding to the ligandseparated by repeated washing of the bead (e.g., magnetic separation) orcolumn. Furthermore, in some aspects a target ligand may be labeled suchas with a fluorophor, a radioisotope or an enzyme. Thus, bacterial cellsmay, in some cases, be selected by detecting a label on a bound ligand.For example, a fluorophore may be used to select cells usingfluorescence activated cell sorting (FACS). Furthermore, in someaspects, bacterial cells may be selected based on binding or lack ofbinding two or more ligands. For instance, bacteria may be selected thatdisplay antibodies that bind to two ligands, wherein each ligand is usedto select the bacterial sequentially. Conversely, in certain aspects,bacteria may be selected that display antibodies that bind to one ligand(such as a ligand comprising a first label) but not to a second ligand(e.g., comprising a second label). The foregoing method may be used, forexample, to identify antibodies that bind to a specific ligand from afamily of two or more related ligands.

In further embodiments, methods for producing bacteria of the invention,may comprising at least two rounds of selection (step iv) wherein thesub-population of bacterial cells obtained in the first round ofselection is subjected to at least a second round of selection based onthe binding of the candidate antibody to target ligand. Furthermore insome aspects the sub-population of bacterial cells obtained in the firstround of selection may be grown under permissive conditions prior to asecond selection (to expand the total number of cells). Thus, in someaspects, methods of the invention may comprise 2, 3, 4, 5, 6, 7, 8, 9,10 or more rounds of selection. Furthermore, in some aspects, asub-population of bacterial cells obtained from each round of selectionwill be grown under permissive conditions before a subsequent round ofselection. In some cases, selection will be performed after removingtarget ligand that is not bound to the antibody. Furthermore, in somecases the stringency of selection may be modified by adjusting the pH,salt concentration, or temperature of a solution comprising bacteriathat display antibodies. Thus, in some aspects, it may be preferred thata bacterial cell of the invention is grown at a sub-physiologicaltemperature such as at about 25° C.

A target ligand for use according to the invention may be any moleculethat can be bound by an antibody. For example, the ligand may be acarbohydrate, a lipid moiety, a polypeptide, an organic polymer, a smallmolecule, or mixture thereof. In some aspects a target ligand may be acancer-associated protein, a cell surface protein, an enzyme, a virus, aglycoprotein or a cell receptor ligand. Methods for labeling a targetligand are well know in the art and may comprise covalent ornon-covalent modification a ligand (see U.S. Pat. No. 7,094,571).

In still further aspects, a method of producing a bacterial cellaccording to the invention may be further defined as a method ofproducing a nucleic acid sequence encoding an antibody having a specificaffinity for a target ligand. Thus, a bacterial cell produced by themethods herein may be used to clone a nucleic acid sequence encoding thecandidate antibody having a specific affinity for a target ligand.Method for isolating and amplifying such a nucleic acid from a cell forexample by PCR are well known in the art and further described below.Thus, a nucleic acid sequence produced by the forgoing methods isincluded as part of the instant invention. Furthermore, such a sequencemay be expressed in a cell to produce an antibody having a specificaffinity for a target ligand. Thus, in some aspects, the inventionprovides a method for producing an antibody having a specific affinityfor a target ligand. Furthermore, the invention includes antibodiesproduced by the methods of the invention. It will be understood howeverthat the antibody variable VH and VL domains produced by such a screenmay be moved into a different antibody type (e.g., comprising adifferent constant regions) and these antibodies are also included aspart of the invention. Thus, in some aspects the invention provides anantibody VH and VL polypeptide produced by the methods of the invention.For example, in some aspects, the invention provides the VH and VLpolypeptides of PA binding antibodies such as YMB2, YMD1, YMF10, YMC3 orYMH2. Furthermore the invention provides intact PA binding antibodiessuch as YMB2, YMD1, YMF10, YMC3 or YMH2.

In a further embodiment the invention provides a bacteriophagecomprising (a) at least one coat protein fused to an antibody-bindingdomain wherein the antibody-binding domain is displayed on the surfaceof the bacteriophage particle. For example, in certain aspects abacteriophage of the invention may be a filamentous bacteriophage suchas M13. Thus, in some cases an antibody-binding domain is fused to a p3coat protein, such as an antibody-binding domain fused to C-terminus ofthe p3 protein. An antibody-binding domains for display on phageparticles may be any of those described herein such as theantibody-binding domain from one of the polypeptides in Table 1. In somepreferred aspects, an antibody-binding domain is an Fc binding domainsuch as an IgG Fc binding domain. Furthermore, an antibody-bindingdomain may comprise a mammalian, bacterial or synthetic binding domain,such as the synthetic ZZ polypeptide. Furthermore, a nucleic acidsequence encoding a bacteriophage coat protein fused to anantibody-binding domain is also included as part of the invention. Forinstance, a nucleic acid sequence may comprise a infectious cloneencoding a bacteriophage coat protein fused to an antibody-bindingdomain along with all of the other essential genes for phage production(e.g., as exemplified by the fUSE5-ZZ vector).

In still further embodiments there is provided a bacteriophagecomprising (a) at least one coat protein fused to an antibody-bindingdomain wherein the antibody-binding domain is displayed on the surfaceof the bacteriophage and (b) a nucleic acid sequence encoding anantibody wherein the nucleic acid sequence is packaged or encapsulatedin the bacteriophage. A nucleic acid sequence encoding an antibody maybe any of those described herein, for example, a nucleic acid comprisingan Ig light chain (L), an Ig heavy (H) chain or both. For instance, anIg heavy and light chain may be encoded by a dicistronic expressionvector such as pMAZ 360-IgG. Preferably, an antibody encoded by such anucleic acid sequence is an intact antibody.

In a further embodiment there is provided a bacteriophage of theinvention further comprising an antibody bound to the antibody-bindingdomain displayed on the surface of the bacteriophage. Preferably, abacteriophage that displays a bound antibody comprises a nucleic acidsequence encoding an antibody packaged within the phage. Thus, in somehighly preferred embodiments, the antibody bound (to theantibody-binding domain) on the surface of the bacteriophage is the sameantibody encoded by the nucleic acid sequence packaged in thebacteriophage. Methods for making such a bacteriophage are exemplifiedherein for instance see FIG. 14. In some aspects such a method maycomprise (a) obtaining a bacteriophage wherein the genome of the phagecomprise sequences encoding a coat protein fused to an antibody-bindingdomain such that the antibody-binding domain is displayed on the surfaceof the bacteriophage; (b) infecting a bacterial cell comprising anantibody expressed in the periplasm and an antibody expression vectorwherein the antibody expression vector comprises a phage packagingsignal; and (c) allowing the phage to replicate thereby producing phageantibody complexes with an encapsulated antibody expression vector.

In still a further embodiment a bacteriophage of the invention may befurther defined as a population of bacteriophages. Such a population maycollectively comprise a plurality of nucleic acid sequences encodingdistinct antibodies (packed within the phage). Thus, in some aspects, apopulation of bacteriophages according to the invention may be definedas comprising at least about 1×10³, 1×10⁴, 1×10⁵, 1×10⁶, 1×10⁷, 1×10⁸,or more distinct antibodies and/or antibody expression vectors.Preferable at least about 90%, 95%, 98%, 99%, 99.9% or more ofbacteriophages in a population that comprise an antibody expressionvector display the same antibody encoded by the vector on their surface.

In yet a further embodiment there is provided a method of producing abacteriophage comprising an antibody having specific affinity for atarget ligand. The foregoing method may comprise (i) obtaining apopulation of bacteriophages comprising (a) at least one coat proteinfused to an antibody-binding domain wherein the antibody-binding domainis displayed on the surface of the bacteriophages, (b) a plurality ofnucleic acid sequences encoding distinct antibodies wherein the nucleicacid sequences are packaged in the bacteriophages and the encodedantibodies are displayed on the surface of the bacteriophages. Next,(ii) the bacteriophages are contacted with a target ligand underconditions wherein the target ligand contacts the antibody, and (iii) abacteriophage(s) is selected based on the binding of the distinctantibody to target ligand thereby producing a bacterial cell. Theskilled artisan will readily understand that in some cases thebacteriophage(s) produced may be amplified by allowing the isolatedphages to propagate in permissive bacterial cells. Furthermore, in someaspects the propagation of said bacteriophage(s) may be facilitated bythe use of a helper phage or in bacterial cells that encode phageproteins in trans. Preferably bacteriophages for use in methods of theinvention comprise an expression vector encoding a plurality of distinctantibodies wherein an expression vector encoding the displayed antibodyis encapsulated within the phage. Bacteriophages produced by the methodsof the invention are included as part of the instant invention.

A target ligand for use according to the invention may be any moleculethat may be bound by an antibody as described herein. The skilledartisan will understand that methods for selecting bacteriophages basedupon their interaction (binding) with a ligand are well known in theart. For example, a ligand may be immobilized on a column or bead (e.g.,a magnetic bead) and the bacteriophages binding to the ligand separatedby repeated washing of the bead (e.g., magnetic separation) or column.Furthermore, in some aspects, bacteriophages may be selected based onbinding or lack of binding two or more ligands. For instance,bacteriophages may be selected that display an antibody that binds totwo ligands. Conversely, in certain aspects, bacteriophages may beselected that display antibodies that bind to one ligand but not to asecond ligand. For example, bacteriophages may be selected based uponbinding or lack of binding to an immobilized ligand and then subjectedto a second selection for binding or lack of binding using a secondimmobilized ligand. The foregoing method may be used, for example, toidentify antibodies that bind to a specific ligand from a family of twoor more related ligands.

In further embodiments, methods for producing bacteriophages of theinvention, may comprise at least two rounds of selection (step iii)wherein the sub-population of bacteriophages obtained in the first roundof selection is subjected to at least a second round of selection basedon the binding of the candidate antibody to target ligand. Furthermorein some aspects the sub-population of bacteriophages obtained in thefirst round of selection may be grown under permissive (e.g., inbacterial cells that may comprise phage trans genes or be coinfectedwith helper phage) conditions prior to a second selection (to expand thetotal number of cells). Thus, in some aspects, methods of the inventionmay comprise 2, 3, 4, 5, 6, 7, 8, 9, 10 or more rounds of selection.Furthermore, in some aspects, the sub-population of bacteriophagesobtained for a round of selection may be grown under permissiveconditions prior to subsequent round of selection. Furthermore, in somecases the stringency of selection may be modified by adjusting the pH,salt content, or shear force (flow rate) or temperature in a solutioncomprising bacteriophages that display antibodies.

In still further aspects, the invention concerns a method of producing abacteriophage according to the invention, wherein the bacteriophagecomprises a nucleic acid sequence encoding an antibody having a specificaffinity for a target ligand. Thus, a bacteriophage produced by themethods herein may be used to clone a nucleic acid sequence encoding adistinct antibody having a specific affinity for a target ligand.Methods for isolating and amplifying such a nucleic acid from abacteriophage or bacteriophage-infected cell (e.g., by PCR) are wellknown in the art. Thus, a nucleic acid sequence produced by theforegoing methods is included as part of the instant invention.Furthermore, such a nucleic acid sequence may be expressed in a cell toproduce an antibody having a specific affinity for a target ligand.Thus, in some aspects, the invention provides a method for producing anantibody having a specific affinity for a target ligand. Furthermore,the invention includes antibodies produced by the methods of theinvention. It will be understood however that the antibody variable VHand VL domains produced by such a screen may be moved into a differentantibody type (e.g., comprising different constant regions) and theseantibodies are also included as part of the invention. Thus, in someaspects the invention provides an antibody VH and VL polypeptideproduced by the methods of the invention.

Embodiments discussed in the context of a methods and/or composition ofthe invention may be employed with respect to any other method orcomposition described herein. Thus, an embodiment pertaining to onemethod or composition may be applied to other methods and compositionsof the invention as well.

As used herein the terms “encode” or “encoding” with reference to anucleic acid are used to make the invention readily understandable bythe skilled artisan however these terms may be used interchangeably with“comprise” or “comprising” respectively.

As used herein the specification, “a” or “an” may mean one or more. Asused herein in the claim(s), when used in conjunction with the word“comprising”, the words “a” or “an” may mean one or more than one.

The use of the term “or” in the claims is used to mean “and/or” unlessexplicitly indicated to refer to alternatives only or the alternativesare mutually exclusive, although the disclosure supports a definitionthat refers to only alternatives and “and/or.” As used herein “another”may mean at least a second or more.

Throughout this application, the term “about” is used to indicate that avalue includes the inherent variation of error for the device, themethod being employed to determine the value, or the variation thatexists among the study subjects.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description. It should beunderstood, however, that the detailed description and the specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and areincluded to further demonstrate certain aspects of the presentinvention. The invention may be better understood by reference to one ormore of these drawings in combination with the detailed description ofspecific embodiments presented herein.

FIG. 1. A schematic diagram showing an example of an antibody screeningmethod of the invention.

FIG. 2. A schematic diagram depicting the pMAZ 360-IgG vector.

FIG. 3. Antibodies can be efficiently produced and purified frombacterial cells. Upon IPTG induction E. coli JUDE1 cells efficientlyproduced soluble intact antibodies as shown by SDS/PAGE Western blotunder non-reducing (FIG. 3A) or reducing conditions (FIG. 3B).Furthermore, IgG antibodies may be efficiently isolated by protein-Apurification as shown by SDS/PAGE and staining analysis undernon-reducing (FIG. 3C) or reducing conditions (FIG. 3D).

FIG. 4A-D. Binding of IgG to spheroplasts displaying the protein ZZdomain. Spheroplasts expressing either the NlpA-ZZ fusion orNlpA-80R(scFv) as a control were labeled with IgG-FITC and were analyzedby FACS. The data show that spheroplasts displaying the ZZ protein as anNlpA fusion specifically bind and become decorated with human IgG. FIG.4A, Fluorescence histogram of spheroplasts expressing NlpA-ZZ andincubated with Human IgG-FITC; FIG. 4B, NlpA-80R+Human IgG-FITC(negative control); FIG. 4C, NlpA-ZZ+purified recombinant M18.1 IgGtogether with its fluorescently labeled antigen, PA-FITC; FIG. 4D,NlpA-80R+purified recombinant M18.1 IgG together with its fluorescentlylabeled antigen, PA-FITC.

FIG. 5. IgG antibodies remain stably associated with ZZ. Fluorescentlylabeled IgG was incubated with spheroplasts comprising membrane anchoredZZ. Cell fluorescence was determined at various time intervals by FACS.Y-axis indicated number of events (cells), x-axis indicated fluorescenceintensity and z-axis indicates incubation time.

FIG. 6A-D. Flow cytometry (FC) analysis of spheroplasts co-expressingthe NlpA-ZZ and recombinant IgG in the same host cell. E. Coli JUDE-1expressing NlpA-ZZ cells were transformed with plasmids pMAZ360-M18.1Hum-IgG (filled) and pMAZ360-26.10-IgG (clear), protein synthesisinduced, the cells were converted to spheroplasts and then labeled with(FIG. 6A) PA-FITC or with (FIG. 6B) digoxigenin-BODIPY™. To confirm thespecificity of IgG binding, spheroplasts were incubated with 10 foldmolar excess of non-labeled PA (FIG. 6C) or BSA-digoxin (FIG. 6D) for 1h at RT prior to incubation with the fluorescent probe. In FIG. 6C,D,filled area represents the binding without pre-incubation with unlabeledcompetitor while clear area represents the FC signal obtained followedthe pre-incubation with the competitor.

FIG. 7. Bacterial cells displaying an anti-PA antibody can besuccessfully selected from a mixed population. Z-axis indicated celltreatment conditions 26.10 (negative control for FITC-PA binding), M18.1Hum (positive control for PA-FITC binding), 1:100,000 (mixed cellpopulation or cell populations after 1 or 2 selection cycles withPA-FITC. FACS analysis is as indicated in FIG. 5.

FIG. 8A-B. A schematic diagram showing the construction of a VH (FIG.8A) or VH and VL (FIG. 8B) antibody library. FIG. 8A right panel,represent Western-blot analysis of 10 VH library clones selected atrandom. HRP-conjugated goat anti-human IgG was used to detect IgGexpression. The membrane was developed using the renaissance Westernblot chemiluminescence reagent (NEN, USA) according to the supplier'sinstructions. FIG. 8B right panel, represent the same experiment withthe complete VH-VL library.

FIG. 9. Bacteria displaying a library of antibodies may be efficientlyselected for ligand binding according to the invention. The number ofcells that exhibit PA-FITC mediated fluorescence is increased with eachselection cycle (as indicated on the z-axis). FACS analysis is displayedas in FIG. 5.

FIG. 10. Table shows the amino acid sequences of the VH and VL domainsof PA binding antibodies. YMB2, VH is SEQ ID NO: 19 and VL is SEQ ID NO:20. YMD1, VH is SEQ ID NO: 21 and VL is SEQ ID NO: 22. YMF10, VH is SEQID NO: 23 and VL is SEQ ID NO: 24. YMC3, VH is SEQ ID NO: 25 and VL isSEQ ID NO: 26. YMH2, VH SEQ ID NO: 27 and VL is SEQ ID NO: 26.

FIG. 11. Bacterial cells expressing each of the indicated antibodies orlibraries (z-axis) were assessed for PA-FITC fluorescence by FACS. Datais graphed as indicted in FIG. 5.

FIG. 12A-B. Full length IgG antibodies produced by methods of theinvention are efficiently expressed. The five PA binding antibodies wereexpressed in bacteria, purified by protein-A chromatography and analyzedby SDS/PAGE and gel staining under non-reducing (FIG. 12A) or reducingconditions (FIG. 12B). In each case, gels were visualized by stainingwith GELCODE® Blue, lane 1: Clone YMB2; lane 2: Clone YMC3; lane 3:Clone YMD1; lane 4: Clone YMF10; lane 5: Clone YMH2; lane 6: standardcommercial human IgG1.

FIG. 13A-B. Analysis of binding affinities of the selected anti-PA IgGclones. FIG. 13A, a table of binding kinetics acquired by SPR. FIG. 13B,an example of kinetic analysis of YMF10 IgG toward PA-63 using the IgGcapture method. PA-63 was injected in duplicate at concentrations of 15,5, 1.5 and 0 nM over YMF10 IgG captured by anti human IgG1 Fcγ fragmentspecific antibody. Approximately 200 RU of IgG was captured for eachconcentration of PA-63 analyzed, followed by a 5 minute stabilizationperiod prior to the injection of PA-63 to determine kinetics (inset).Enlarged portion from inset displays association and dissociation ofPA-63. The Y-axis was transformed to a baseline of 0 response unitsprior to the association phase to aid in kinetic analysis.

FIG. 14A-B. A schematic diagram showing antibody phage display methodsof the invention. Bacteria expressing antibodies in the periplasmicspace may be infected with a phage comprising a coat protein fused to anantibody-binding domain (FIG. 14A) thereby producing phage that displaysthe antibody and comprises a nucleic acid encoding the antibody (FIG.14B).

FIG. 15A-B. Bacteriophages displaying antibodies efficiently bind toantibody ligands and compete with free IgG. Phage particles displayingeither anti-PA or anti-Digoxin antibody specifically bound to ELISAplates coated with PA (FIG. 15A) or Digoxin-BSA (FIG. 15B).

FIG. 16A-B. Bacteriophages displaying antibodies to a target ligand canbe efficiently produced by methods of the invention. Mixed phagespopulations can be enriched for phages displaying anti-PA oranti-Digoxin antibodies by panning with the appropriate ligand asindicated. Figure indicated optical density by ELISA with plates coatedwith PA (FIG. 16A) or Digoxin (FIG. 16B).

FIG. 17. The productivity of the semi-synthetic library can be evaluatedby monitoring the expression of fully assembled IgGs in the E. coliperiplasm by analyzing library clones selected at random. HRP-conjugatedgoat anti-human IgG was used to detect IgG expression. The membrane wasdeveloped using the Renaissance® Western blot chemiluminescence reagent(NEN, USA) according to the supplier's instructions. Top arrow indicatesfolded fully assembled hetero-tetramer IgG1.

FIG. 18A-B. Representative flow cytometry data. (A) Control JUDE-1 cellsexpressing only NlpA-ZZ proteins, double-labeled with FITC-conjugatedHuman Annexin V and Alexa Flour 647-Chicken anti-Human IgG Fc. (B)library cells labeled for the initial round of FACS sorting. Cells aredouble-labeled with FITC-conjugated Human Annexin V and Alexa Flour647-Chicken anti-Human IgG Fc to evaluate for antigen binding and IgGexpression, respectively. The thick outline indicates a typical sortgate collecting approximately the top 5% of the population displayingthe highest fluorescent intensities.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention presents a completely novel concept that enablesthe screening of intact libraries of aglycosylated IgG proteins inbacteria. In all the known display technologies the protein of interestmust be expressed as either a C- or N-terminal fusion to a displayprotein. However, this requirement imposes significant limitations thatcan adversely affect protein function or stability. As a result, many ofthe proteins that are isolated following library screening can only foldin the context of a fusion protein and cannot be expressedindependently. In the present invention the IgG antibodies are initiallyexpressed in their native form as soluble non-fusion proteins in theperiplasmic space of E. coli. Consequently these proteins are notselected to fold or be expressed together with a fusion partner proteinas is the case with earlier display technologies.

Furthermore, the methodology described herein overcomes the requirementsof post-isolation manipulation related with antibody fragments prior toincreasing their affinity/stability/avidity characteristics, therebyfacilitating the isolation of antibody clones with a broad range ofaffinities to antigens of interest. Finally, in the present inventionthe IgG library screening vector is also utilized as the expressionvector. Therefore, isolated antibodies can be readily expressed inbacteria and employed for target binding and neutralization in animalexperiments directly. This feature is likely to represent important timesavings for the pharmaceutical industry.

In this invention the efficient expression of soluble fully assembledintact IgG antibody libraries in E. coli periplasm was carried out byconstructing the pMAZ360-IgG expression vector for convenient cloning ofVH and Vκ domains linked to human γ1 and κ constant domains,respectively (FIG. 2). This plasmid also contains the packaging signalof f1 that enables the packaging of the plasmid as ssDNA in the presenceof helper phage. Libraries of IgG antibodies expressed in E. coli havenow been screened using two different approaches: (i) microbial displayand (ii) phage display. For both screening strategies the IgG moleculesin the periplasm are captured by cells or by phage, respectively. In oneembodiment the IgG is capture by cells or phage via an IgG bound proteinthat is anchored onto an exposed site on the cell or phage,respectively. It has also been shown however, that IgG can be capturedby bacterial cells via non-specific interactions with cell wallcomponents. Thus as a result, IgG molecules remain associated with cellsand can bind antigen even when the outer membrane of the bacteria hasbeen removed.

In one embodiment, cells expressing full length IgG in the periplasm arebound to the outer surface of the periplasmic membrane of E. coli orother Gram negative bacteria by means of an IgG binding protein. ManyIgG binding proteins have been described in the literature and can beused for the purpose of capturing IgG (Table 1). A preferred embodimentutilizes an engineered S. aureus Protein A domain called the ZZ domainwhich binds to the Fc-region of IgG. The ZZ domain or any other suitableIgG binding protein is expressed as a fusion protein that renders itanchored or strongly associated with components of the cell envelope.Anchoring or strong association with the cell envelope is such that,upon removal of the outer membrane by chemical, environmental or genetictreatment the IgG binding protein remains stably attached to the cell.As an example, a method for anchoring an IgG protein onto the cellenvelope involves the fusion of said protein with the leader peptide andsix amino acid N-terminal portion of an E. coli lipoprotein. One suchsequence is (SEQ ID NO:4) derived from the E. coli lipoprotein NlpA.Fusions to this sequence become fatty acylated in vivo and as a resultthey are anchored to the membrane via the fatty acid moiety. Theassociation of lipoproteins with the membrane is very stable and is notdisrupted unless the cells are treated with strong detergents or withenzymes which result in cell lysis. While this sequence (SEQ ID NO:4)serves as an example it should be understood that any other sequencecapable of providing a strong association between the cell and the IgGanchoring protein, whether covalent or non-covalent, can be used for thepurposes described henceforth in this application.

In one embodiment, IgG antibodies are cloned into the pMAZ360-IgGexpression vector for periplasmic expression (SEQ ID NO:1). This vectordirects the expression of both the heavy and light chains form the samepromoter. The heavy and light chains are secreted into the periplasmwhere they fold and associate to form a tertameric IgG molecule.Biochemical analysis has revealed that such molecules are completelyfunctional and the IgG exhibits the expected binding affinity toantigen.

The IgG molecules are expressed in soluble form in the periplasm. Inother words the polypeptide product is an intact IgG which does notcontain any extraneous amino acids or is fused to unrelatedpolypeptides. In the periplasm the IgG interacts with and becomes boundto the IgG-binding protein which, as described above is in turnexpressed in a form that remains associated with the cell. The next stepis treatment of the cells to disrupt the outer membrane which normallyserves as a barrier that restricts access of the periplasmic IgG toexternally added antigens within the extracellular fluid. Disruption ofthe outer membrane normally results in the leakage of periplasmicproteins from the cells. However, in this case the IgG protein is boundonto the cell by the cell envelop-attached IgG binding protein. In thepreferred embodiment, the membrane anchored (NlpA-ZZ) fusion binds tothe periplasmic IgG which thus remains cell-associated even after theouter membrane has been disrupted or even completely removed.

Once the outer membrane has been disrupted or removed the cells are thenincubated with fluorescently labeled antigen. Cells expressing IgGantibodies that recognize the antigen become fluorescent. Thefluorescent cells can be readily distinguished by fluorescence activatedcell sorting (FACS). FACS is a very powerful technique for libraryscreening purposes (Georgiou, 2000). FACS can be used to screenlibraries of billions of bacterial cells with high efficiency.

A surprising finding by the inventors is that the binding of NlpA-ZZ toIgG is kinetically very stable. The ZZ domain exhibits a modest affinityfor IgG in solution (Nilsson et al., 1987; Hober et al., 2006). However,the inventors found that cells expressing NlpA-ZZ bind and retainperiplasmically expressed IgG antibodies for very long times. In oneexample the inventors found that NlpA-ZZ displaying cells that werefirst incubated with low concentration of fluorescently-labeled IgGs andthen subjected to extensive washing steps, retained stable fluorescencesignal for as long as three consecutive days. In another example theinventors determined the binding equilibrium of the NlpA-ZZ-IgG complexby means of direct cell-ELISA and found that the apparent affinity ofthe NlpA-ZZ moiety to the Fc of IgGs was in the sub-nanomolar range. Theaffinity obtained differs by approximately two orders of magnitude fromthe value for the interaction of the two proteins in solution. Thisdifference arises from avidity effects from the high density of NlpA-ZZfusion proteins expressed on the periplasmic membrane.

A second screening method for the isolation of IgG antibodies fromcombinatorial libraries utilizes phage display. In this embodiment theIgG molecules are expressed as soluble, proteins in E. coli periplasmusing the expression vector pMAZ360-IgG described herein. Cellsexpressing IgG are simultaneously infected by FUSE5-ZZ phage particles(Yacoby et al., 2006). These phage particles allow polyvalent display ofthe IgG-binding protein ZZ, on all copies of the p3 minor coat proteinof filamentous bacteriophage. In other words the ZZ moiety isgenetically fused to the g3 gene located on the phage genome. Therefore,phage particles assembled in the E. coli periplasm covalently displaythe ZZ moiety or any other IgG binding protein on all 3-5 copies of thephage p3 coat protein. In one example, intact IgGs becomes bound p3-ZZin the periplasm and then the p3-ZZ-IgG complex becomes incorporatedinto phage to form FUSE5-ZZ-IgG phage particles. In the preferredembodiment the pMAZ360-IgG expression vector performs also as phagemid.A phagemid is defined as a plasmid containing the packaging andreplication origin of the filamentous bacteriophage. This latterproperty allows the packaging of the phagemid into a phage coat when itis present in an E. coli host strain infected with a filamentous phage(superinfection). In the preferred embodiment FUSE5-ZZ-IgG phageparticles preferentially to package the high copy number pMAZ360-IgGphagemid over the replication-defective FUSE5-ZZ genome (Scott andSmith, 1990). Next, packaged particles produced, harboring the phagemid,display the p3-ZZ-IgG fusion protein on the surface of the particles andare secreted into the medium. Such packaged particles are able to injecttheir phagemid into a new host bacterium, where they can be propagated.The special property of the system lies in the fact that since thepackaging takes place in individual cells usually infected by a singlevariant FUSE5-ZZ phage, the particles produced on propogation containthe gene encoding the particular IgG variant displayed on the particle'ssurface. Several cycles of affinity selection for clones exhibiting therequired binding properties, e.g., binding to a particular targetmolecule immobilized on a surface, followed by amplification of theenriched clones leads to the isolation of antigen-specific IgGantibodies. The inventors found that the IgG captured by theZZ-displaying phage is kinetically very stable. Competition of the boundIgGs by excess of soluble IgG competitors resulted in a very mildsubstitution. The inventors attribute the stable interaction of thecomplex ZZ-IgG to the character of the FUSE5-ZZ phage that display theZZ moiety on all copies of the phage p3 coat protein. Hence the adjacentZZ molecules contribute by avidity manners to capture and retain the IgGmolecules in a very stable way. It should be understood that in additionto filamentous bacteriophages described herein, other bacterial phagesincluding lytic phages such as lambda and the T series phages can beemployed for the display of IgG antibodies and for the screening of IgGlibraries.

I. Anchored Periplasmic Expression

In some aspects of the invention antibodies are expressed in theperiplasmic space bound to antibody-binding domains that serve asanchors to the periplasmic face of the inner membrane. Such a techniquemay be termed “Anchored Periplasmic Expression” or “APEx”.

The periplasmic compartment is contained between the inner and outermembranes of Gram negative cells (see, e.g., Oliver, 1996). As asub-cellular compartment, it is subject to variations in size, shape andcontent that accompany the growth and division of the cell. Within aframework of peptidoglycan heteroploymer is a dense mileau ofperiplasmic proteins and little water, lending a gel-like consistency tothe compartment (Hobot et al., 1984; van Wielink and Duine, 1990). Thepeptidoglycan is polymerized to different extents depending on theproximity to the outer membrane, close-up it forms the murein sacculusthat affords cell shape and resistance to osmotic lysis.

The outer membrane (see Nikaido, 1996) is composed of phospholipids,porin proteins and, extending into the medium, lipopolysaccharide (LPS).The molecular basis of outer membrane integrity resides with LPS abilityto bind divalent cations (Mg²⁺ and Ca²⁺) and link each otherelectrostatically to form a highly ordered quasi-crystalline ordered“tiled roof” on the surface (Labischinski et al., 1985). The membraneforms a very strict permeability barrier allowing passage of moleculesno greater than around 650 Da (Burman et al., 1972; Decad and Nikaido,1976) via the porins. The large water filled porin channels areprimarily responsible for allowing free passage of mono anddisaccharides, ions and amino acids in to the periplasm compartment(Nikaido and Nakae, 1979; Nikaido and Vaara, 1985). With such strictphysiological regulation of access by molecules to the periplasm it mayappear, at first glance, inconceivable that APEx should work unless theligands employed are at or below the 650 Da exclusion limit or areanalogues of normally permeant compounds. However, the inventors haveshown that ligands greater than 2000 Da in size can diffuse into theperiplasm without disruption of the periplasmic membrane. Such diffusioncan be aided by one or more treatments of a bacterial cell, therebyrendering the outer membrane more permeable, as is described hereinbelow.

II. Permeabilization of the Outer Membrane

In one embodiment of the invention, methods are employed for increasingthe permeability of the outer membrane to one or more labeled ligand.This can allow screening access of labeled ligands otherwise unable tocross the outer membrane. However, certain classes of molecules, forexample, hydrophobic antibiotics larger than the 650 Da exclusion limit,can diffuse through the bacterial outer membrane itself, independent ofmembrane porins (Farmer et al., 1999). The process may actuallypermeabilize the membrane on so doing (Jouenne and Junter, 1990). Such amechanism has been adopted to selectively label the periplasmic loops ofa cytoplasmic membrane protein in vivo with a polymyxin B nonapeptide(Wada et al., 1999). Also, certain long chain phosphate polymers (100Pi) appear to bypass the normal molecular sieving activity of the outermembrane altogether (Rao and Torriani, 1988).

Conditions have been identified that lead to the permeation of ligandsinto the periplasm without loss of viability or release of the expressedproteins from the cells, but the invention may be carried out withoutmaintenance of the outer membrane. By anchoring candidate bindingpolypeptides to the outer side of the inner (cytoplasmic) membrane usingfusion polypeptides, the need for maintenance of the outer membrane (asa barrier to prevent the leakage of the biding protein from the cell) todetect bound labeled ligand is removed. As a result, cells expressingbinding proteins anchored to the outer (periplasmic) face of thecytoplasmic membrane can be fluorescently labeled simply by incubatingwith a solution of fluorescently labeled ligand in cells that eitherhave a partially permeabilized membrane or a nearly completely removedouter membrane.

The permeability of the outer membrane of different strains of bacterialhosts can vary widely. It has been shown previously that increasedpermeability due to OmpF overexpression was caused by the absence of ahistone like protein resulting in a decrease in the amount of a negativeregulatory mRNA for OmpF translation (Painbeni et al., 1997). Also, DNAreplication and chromosomal segregation is known to rely on intimatecontact of the replisome with the inner membrane, which itself contactsthe outer membrane at numerous points. A preferred host for libraryscreening applications is E. coli ABLEC strain, which additionally hasmutations that reduce plasmid copy number.

The inventors have also noticed that treatments such as hyperosmoticshock can improve labeling significantly. It is known that many agentsincluding, calcium ions (Bukau et al., 1985) and even Tris buffer (Irvinet al., 1981) alter the permeability of the outer-membrane. Further, theinventors found that phage infection stimulates the labeling process.Both the filamentous phage inner membrane protein pIII and the largemultimeric outer membrane protein pIV can alter membrane permeability(Boeke et al., 1982) with mutants in pIV known to improve access tomaltodextrins normally excluded (Marciano et al., 1999). Using thetechniques of the invention, comprising a judicious combination ofstrain, salt and phage, a high degree of permeability was achieved(Daugherty et al., 1999). Cells comprising anchored binding polypeptidesbound to fluorescently labeled ligands can then be easily isolated fromcells that express binding proteins without affinity for the labeledligand using flow cytometry or other related techniques. However, itwill typically be desired to use less disruptive techniques in order tomaintain the viability of cells. EDTA and Lysozyme treatments may alsobe useful in this regard.

III. Anchored Antibody-Binding Domains

In one embodiment of the invention, bacterial cells are providedexpressing fusion polypeptides on the outer face of the inner membrane.Such a fusion polypeptide may comprise a fusion between anantibody-binding domain and a polypeptide serving as an anchor to theouter face of the inner membrane. It will be understood to those ofskill in the art that additional polypeptide sequences may be added tothe fusion polypeptide and not depart from the scope of the invention.One example of such a polypeptide is a linker polypeptide serving tolink the anchor polypeptide and the antibody-binding domain. The generalscheme behind the invention comprises the advantageous expression of aheterogeneous collection of candidate binding polypeptides.

Anchoring to the inner membrane may be achieved by use of the leaderpeptide and the first six amino acids of an inner membrane lipoprotein.One example of an inner membrane lipoprotein is NlpA (new lipoproteinA). The first six amino acid of NlpA can be used as an N terminal anchorfor protein to be expressed to the inner membrane. NlpA was identifiedand characterized in Escherichia coli as a non-essential lipoproteinthat exclusively localizes to the inner membrane (Yu, 1986; Yamaguchi,1988).

As with all prokaryotic lipoproteins, NlpA is synthesized with a leadersequence that targets it for translocation across the inner membrane viathe Sec pathway. Once the precursor protein is on the outer side of theinner membrane the cysteine residue of the mature lipoprotein forms athioether bond with diacylglyceride. The signal peptide is then cleavedby signal peptidase II and the cysteine residue is aminoacylated(Pugsley, 1993). The resulting protein with its lipid modified cysteineon its N terminus can then either localize to the inner or outermembrane. It has been demonstrated that this localization is determinedby the second amino acid residue of the mature lipoprotein (Yamaguchi,1988). Aspartate at this position allows the protein to remain anchoredvia its N terminal lipid moiety to the inner membrane, whereas any otheramino acid in the second position generally directs the lipoprotein tothe outer membrane (Gennity and Inouye, 1992). This is accomplished byproteins LolA, LolB and the ATP dependant ABC transporter complex LolCDE(Yakushi, 2000, Masuda 2002). NlpA has aspartate as its second aminoacid residue and therefore remains anchored within the inner membrane.

It has been reported that by changing amino acid 2 of lipoproteins to anArginine (R) will target them to reside in the inner membrane (Yakushi,1997). Therefore all lipoproteins in E. coli (and potentially other Gramnegative bacteria) can be anchor sequences. All that is required is asignal sequence and an arginine at amino acid 2 position. This constructcould be designed artificially using an artificial sec signal sequencefollowed by the sec cleavage region and coding for cysteine as aminoacid 1 and arginine as amino acid 2 of the mature protein. Transmembraneproteins could also potentially be used as anchor sequences althoughthis will require a larger fusion construct.

Examples of anchors that may find use with the invention includelipoproteins, Pullulanase of K. pneumoniae, which has the CDNSSS maturelipoprotein anchor, phage encoded celB, and E. coli acrE (envC).Examples of inner membrane proteins which can be used as protein anchorsinclude: AraH, MglC, MalF, MalG, Mal C, MalD, RbsC, RbsC, ArtM, ArtQ,GlnP, ProW, HisM, HisQ, LivH, LivM, LivA, LivE, DppB, DppC, OppB, AmiC,AmiD, BtuC, FhuB, FecC, FecD, FecR, FepD, NikB, NikC, CysT, CysW, UgpA,UgpE, PstA, PstC, PotB, PotC, PotH, PotI, ModB, NosY, PhnM, LacY, SecY,TolC, Dsb, B, DsbD, TonB, TatC, CheY, TraB, ExbD, ExbB and Aas. Further,a single transmembrane loop of any cytoplasmic protein can be used as amembrane anchor.

The preparation of diverse populations of fusion proteins in the contextof phage display is known (see, e.g., U.S. Pat. No. 5,571,698). Thus,these techniques may be used to generate a fusion protein between aphage coat protein and an antibody-binding domain. Similar techniquesmay be employed with the instant invention by linking the protein ofinterest to an anchor for the periplasmic face of the cytoplasmicmembrane instead of, for example, the amino-terminal domain of the geneIII coat protein of the filamentous phage M13, or anothersurface-associated molecule. Such fusions can be mutated to form alibrary of structurally related fusion proteins that are expressed inlow quantity on the periplasmic face of the cytoplasmic membrane inaccordance with the invention. As such, techniques for the creation ofheterogeneous collections of candidate molecules which are well known tothose of skill in the art in conjunction with phage display, can beadapted for use with the invention. Those of skill in the art willrecognize that such adaptations will include the use of bacterialelements for expression of fusion proteins anchored to the periplasmicface of the inner membrane, including, promoter, enhancers or leadersequences. The current invention provides the advantage relative tophage display of not requiring the use of phage or expression ofmolecules on the outer cell surface, which may be poorly expressed ormay be deleterious to the host cell.

Antibody-binding domains are also well known in the art. Such domainsmay be mammalian, bacterial or synthetic domains. For instance Table 1exemplifies some the antibody-binding polypeptides/genes that arecontemplated for use according to the instant invention. However,fragments comprising only the antibody binging sequences of thepolypeptide of Table 1 may also be used according to the invention.

TABLE 1 Selected Antibody-binding Polypeptides Protein name Gene nameDescription Organisms Length (aa) Reference Fc-gamma FCGR2A Low affinityHomo sapiens 317 (Stuart et RII-a immunoglobulin (Human) al., 1987)gamma Fc region receptor II-a precursor Fc-gamma FCGR2A Low affinity Pantroglodytes 316 RII-a immunoglobulin (Chimpanzee) gamma Fc regionreceptor II-a precursor Fc-gamma FCGR2B Low affinity Homo sapiens 310(Stuart et RII-b immunoglobulin (Human) al., 1989) gamma Fc regionreceptor II-b precursor Fc-gamma FCGR2C Low affinity Homo sapiens 323(Stuart et RII-c immunoglobulin (Human) al., 1989) gamma Fc regionreceptor II-c precursor Fc-gamma FCGR3A Low affinity Homo sapiens 254(Ravetch and RIIIa immunoglobulin (Human) Perussia, 1989) gamma Fcregion receptor III-A precursor Fc-gamma FCGR3B Low affinity Homosapiens 233 (Ravetch and RIIIb immunoglobulin (Human) Perussia, 1989)gamma Fc region receptor III-B precursor Fc-gamma FCGR1A High affinityHomo sapiens 374 (Allen and RI) immunoglobulin (Human) Seed, 1988) gammaFc receptor I precursor Fc-gamma Fcgr1 High affinity Mus musculus 404(Sears et RI immunoglobulin (Mouse) al., 1990) gamma Fc receptor Iprecursor Fc-gamma FCGR2 Low affinity Bos taurus 296 (Zhang et RIIimmunoglobulin (Bovine) al., 1994) gamma Fc region receptor II precursorFc-gamma FCGR2 Low affinity Cavia porcellus 341 (Tominaga et RIIimmunoglobulin (Guinea pig) al., 1990) gamma Fc region receptor IIprecursor Fc-gamma Fcgr2 Low affinity Mus musculus 330 (Ravetch et RIIimmunoglobulin (Mouse) al., 1986) gamma Fc region receptor II precursorFc-gamma Fcgr2 Low affinity Rattus 285 (Bocek and RII immunoglobulinnorvegicus (Rat) Pecht, 1993) gamma Fc region receptor II precursorFc-gamma FCGR3 Low affinity Bos taurus 250 (Collins et RIIIimmunoglobulin (Bovine) al., 1997) gamma Fc region receptor IIIprecursor Fc-gamma FCGR3 Low affinity Macaca 254 RIII immunoglobulinfascicularis gamma Fc region (Crab eating receptor III macaque)precursor (Cynomolgus monkey) Fc-gamma Fcgr3 Low affinity Mus musculus261 (Ravetch et RIII immunoglobulin (Mouse) al., 1986) gamma Fc regionreceptor III precursor Fc-gamma FCGR3 Low affinity Sus scrofa (Pig) 257(Halloran et RIII immunoglobulin al., 1994) gamma Fc region receptor IIIprecursor Fc-gamma Fcgr3 Low affinity Rattus 267 (Zeger et RIIIimmunoglobulin norvegicus (Rat) al., 1990) gamma Fc region receptor IIIprecursor FcRn FCGRT IgG receptor Homo sapiens 365 transporter FcRn(Human) large subunit p51 precursor FcRn FCGRT IgG receptor Macaca 365transporter FcRn fascicularis large subunit p51 (Crab eating precursormacaque) (Cynomolgus monkey) FcRn Fcgrt IgG receptor Mus musculus 365(Ahouse et transporter FcRn (Mouse) al., 1993) large subunit p51precursor FcRn Fcgrt IgG receptor Rattus 366 (Simister and transporterFcRn norvegicus (Rat) Mostov, 1989) large subunit p51 precursor MRPprotein mrp4 Fibrinogen- and Streptococcus 388 (Stenberg et Ig-bindingprotein pyogenes al., 1992) precursor Protein B cAMP factorStreptococcus 226 (Ruhlmann et agalactiae al., 1988) protein A spaImmunoglobulin Staphylococcus 516 (Uhlen et G-binding protein aureus(strain al., 1984) A precursor NCTC 8325) protein A spa ImmunoglobulinStaphylococcus 508 (Shuttleworth et G-binding protein aureus al., 1987)A precursor protein A spa Immunoglobulin Staphylococcus 450 (Kuroda etG-binding protein aureus (strain al., 2001) A precursor Mu50/ATCC700699) protein A spa Immunoglobulin Staphylococcus 450 (Kuroda etG-binding protein aureus (strain al., 2001) A precursor N315) protein Gspg Immunoglobulin Streptococcus 448 (Fahnestock et G-binding proteinsp. group G al., 1986) G precursor protein G spg ImmunoglobulinStreptococcus 593 (Olsson et G-binding protein sp. group G al., 1987) Gprecursor protein H Immunoglobulin Streptococcus 376 (Gomi et G-bindingprotein pyogenes al., 1990) H precursor serotype M1 Protein sbi sbiImmunoglobulin Staphylococcus 436 (Zhang et G-binding protein aureus(strain al., 1998) sbi precursor NCTC 8325-4) Allergen Asp Allergen Aspfl 1 Aspergillus 32 fl 1 causes an allergic flavus reaction in human.Binds to IgE and IgG Allergen Asp Allergen Asp fl 2 Aspergillus 20 fl 2causes an allergic flavus reaction in human. Binds to IgE and IgGAllergen Asp Allergen Asp fl 3 Aspergillus 32 fl 3 causes an allergicflavus reaction in human. Binds to IgE and IgG

Methods for creation of fusion proteins are well known to those of skillin the art (see, for example, U.S. Pat. No. 5,780,279). One means fordoing so comprises constructing a gene fusion between a candidatebinding polypeptide and an anchor sequence and mutating the bindingprotein encoding nucleic acid at one or more codons, thereby generatinga family of mutants. The mutated fusion proteins can then be expressedin large populations of bacteria. Those bacteria in which a targetligand binds, can then be isolated and the corresponding nucleic acidencoding the binding protein can be cloned.

IV. Antibody Libraries

Examples of techniques that could be employed in conjunction with theinvention for creation of diverse antibody-binding domain and/orantibodies include the techniques for expression of immunoglobulin heavychain libraries described in U.S. Pat. No. 5,824,520. In this technique,a single chain antibody library is generated by creating highlydivergent, synthetic hypervariable regions. Similar techniques forantibody display are given by U.S. Pat. No. 5,922,545. These techniquesmay be adapted to generate intact or full length antibody libraries asdescribed herein.

In certain embodiments, libraries of semi-synthetic antibodies can beprepared. Semi-synthetic antibodies contain a fully or partiallyrandomized stretch of amino acid within the variable domain of animmunoglobulin heavy chain, light chain, or both. A polypeptide orpolypeptide sequence is “randomized” when it has one or more regions,each containing a fully or partially random amino acid sequence. In afully randomized sequence, all 20 of the naturally occurring,genetically encoded amino acids can be present at each amino acidposition in the random sequence. In a partially randomized sequence,less than 20 of the naturally occurring, genetically encoded amino acidsare present at each amino acid position in the random sequence.

The variable domains of both immunoglobulin heavy and light chainscontain three hypervariable regions called complementarity determiningregions (“CDRs”) which alternate with four conserved regions calledframework regions (“FRs”). The different regions are arranged from N- toC-terminus as FR1-CDR1-FR2-CDR2-FR3-CDR3-FR4. Hypervariable andframework regions can be defined by reference to a conventionalnumbering scheme (the Kabat numbering scheme) and standard textsdescribing antibody structure, e.g., Kabat et al “Sequences of Proteinsof Immunological Interest: 5th Edition” U.S. Department of Health andHuman Services, U.S. Government Printing Office (1991). In certainembodiments of the present invention, the CDR3 region of animmunoglobulin heavy or light chain, or both, is fully or partiallyrandomized

To produce randomized antibodies, double stranded nucleic acid moleculesencoding randomized nucleotide sequences can be prepared and cloned inexpression vectors. A nucleic acid, such as an oligonucleotide orpolynucleotide, or a nucleic acid sequence is randomized when it has oneor more regions, each containing a fully or partially random nucleicacid sequence. A fully randomized nucleic acid sequence is a sequence inwhich each base (A, T, C, and G) is present at each position in therandom sequence. In a partially randomized sequence, less than four ofthe bases are present at each position in the random sequence. Standardsymbols used to designate randomized bases in nucleic acid sequencesare: R=A,G; Y=C,T; M=A,C; K=G,T; W=A,T; S=C,G; B=C,G,T; D=A,G,T;H=A,C,T; V=A,C,G; and N=A,C,G,T.

In essence, a randomized oligonucleotide, polynucleotide or polypeptideis a set of oligonucleotides, polynucleotides or polypeptides,respectively, each member of which corresponding to one of the sequencesmaking up the randomized region. Thus, a randomized antibody isrepresented by a collection of antibodies, each comprising one of theamino acid sequences of the randomized region.

V. Screening Distinct Antibodies

The present invention provides methods for identifying molecules capableof binding a target ligand. The binding polypeptides screened maycomprise large libraries of diverse candidate substances, or,alternatively, may comprise particular classes of compounds selectedwith an eye towards structural attributes that are believed to make themmore likely to bind the target ligand. In one embodiment of theinvention, the candidate binding protein is an antibody, or a fragmentor portion thereof. In other embodiments of the invention, the candidatemolecule may be another binding protein.

To identify a candidate molecule capable of binding a target ligand inaccordance with the invention, one may carry out the steps of: providinga population of Gram negative bacterial cells or phages that display adistinct antibody; admixing the bacteria or phages and at least a firstlabeled or immobilized target ligand capable of contacting the antibodyand identifying at least a first bacterium or phage expressing amolecule capable of binding the target ligand.

In some aspects of the aforementioned method, the binding betweenantibody and a labeled ligand will prevent diffusing out of a bacterialcell. In this way, molecules of the labeled ligand can be retained inthe periplasm of the bacterium comprising a permeablized outer membrane.Alternatively, the periplasm can be removed, whereby the displayedantibody will cause retention of the bound candidate molecule. Thelabeling may then be used to isolate the cell expressing a bindingpolypeptide capable of binding the target ligand, and in this way, thegene encoding the binding polypeptide isolated. The molecule capable ofbinding the target ligand may then be produced in large quantities usingin vivo or ex vivo expression methods, and then used for any desiredapplication, for example, for diagnostic or therapeutic applications, asdescribed below.

Binding polypeptides or antibodies isolated in accordance with theinvention also may help ascertain the structure of a target ligand. Inprinciple, this approach yields a pharmacophore upon which subsequentdrug design can be based. It is possible to bypass proteincrystallography altogether by generating anti-idiotypic antibodies to afunctional, pharmacologically active antibody. As a mirror image of amirror image, the binding site of anti-idiotype would be expected to bean analog of the original antigen. The anti-idiotype could then be usedto identify and isolate peptides from banks of chemically- orbiologically-produced peptides. Selected peptides would then serve asthe pharmacophore. Anti-idiotypes may be generated using the methodsdescribed herein for producing antibodies, using an antibody as theantigen. On the other hand, one may simply acquire, from variouscommercial sources, small molecule libraries that are believed to meetthe basic criteria for binding the target ligand. Such libraries couldbe provided by way of nucleic acids encoding the small molecules orbacteria expressing the molecules.

A. Cloning of Binding Protein Coding Sequences

The binding affinity of an antibody or other binding protein can, forexample, be determined by the Scatchard analysis of Munson & Pollard(1980) or by Surface Plasmon Resonance Spectrometry using commerciallyavailable instruments and microfluidic chips. After a bacterial cell isidentified that produces molecules of the desired specificity, affinity,and/or activity, the corresponding coding sequence may be cloned. Inthis manner, DNA encoding the molecule can be isolated and sequencedusing conventional procedures (e.g., by using oligonucleotide probesthat are capable of binding specifically to genes encoding the antibodyor binding protein).

Once isolated, the antibody or binding protein DNA may be placed intoexpression vectors, which can then be transfected into host cells suchas simian COS cells, Chinese hamster ovary (CHO) cells, or myeloma cellsthat do not otherwise produce immunoglobulin protein, to obtain thesynthesis of binding protein in the recombinant host cells. The DNA alsomay be modified, for example, by substituting the coding sequence forhuman heavy and light chain constant domains in place of the homologousmurine sequences (Morrison, et al., 1984), or by covalently joining tothe immunoglobulin coding sequence all or part of the coding sequencefor a non-immunoglobulin polypeptide. In that manner, “chimeric” or“hybrid” binding proteins are prepared to have the desired bindingspecificity.

Typically, such non-immunoglobulin polypeptides are substituted for theconstant domains of an antibody, or they are substituted for thevariable domains of one antigen-combining site of an antibody to createa chimeric bivalent antibody comprising one antigen-combining sitehaving specificity for the target ligand and another antigen-combiningsite having specificity for a different antigen.

Chimeric or hybrid antibodies also may be prepared in vitro using knownmethods in synthetic protein chemistry, including those involvingcrosslinking agents. For example, immunotoxins may be constructed usinga disulfide exchange reaction or by forming a thioether bond. Examplesof suitable reagents for this purpose include iminothiolate andmethyl-4-mercaptobutyrimidate.

It will be understood by those of skill in the art that nucleic acidsmay be cloned from viable or inviable cells. In the case of inviablecells, for example, it may be desired to use amplification of the clonedDNA, for example, using PCR. This may also be carried out using viablecells either with or without further growth of cells.

B. Maximization of Protein Affinity for Ligands

In a natural immune response, antibody genes accumulate mutations at ahigh rate (somatic hypermutation). Some of the changes introduced willconfer higher affinity, and B cells displaying high-affinity surfaceimmunoglobulin. This natural process can be mimicked by employing thetechnique known as “chain shuffling” (Marks et al., 1992). In thismethod, the affinity of “primary” human antibodies obtained inaccordance with the invention could be improved by sequentiallyreplacing the heavy and light chain V region genes with repertoires ofnaturally occurring variants (repertoires) of V domain genes obtainedfrom unimmunized donors. This technique allows the production ofantibodies and antibody fragments with affinities in the nM range. Astrategy for making very large antibody repertoires was described byWaterhouse et al., (1993), and the isolation of a high affinity humanantibody directly from such large phage library was reported by Griffithet al., (1994). Gene shuffling also can be used to derive humanantibodies from rodent antibodies, where the human antibody has similaraffinities and specificities to the starting rodent antibody. Accordingto this method, which is also referred to as “epitope imprinting”, theheavy or light chain V domain gene of rodent antibodies obtained by thephage display technique is replaced with a repertoire of human V domaingenes, creating rodent-human chimeras. Selection of the antigen resultsin isolation of human variable regions capable of restoring a functionalantigen-binding site, i.e. the epitope governs (imprints) the choice ofpartner. When the process is repeated in order to replace the remainingrodent V domain, a human antibody is obtained (see PCT patentapplication WO 93/06213, published Apr. 1, 1993). Unlike traditionalhumanization of rodent antibodies by CDR grafting, this techniqueprovides completely human antibodies, which have no framework or CDRresidues of rodent origin.

C. Labeled Ligands

In one embodiment of the invention, an antibody or binding protein isisolated which has affinity for a labeled ligand. By permeabilizationand/or removal of the periplasmic membrane of a Gram negative bacteriumin accordance with the invention, labeled ligands of potentially anysize could be screened. In the absence of removal of the periplasmicmembrane, it will typically be preferable that the labeled ligand isless that 50,000 Da in size in order to allow efficient diffusion of theligand across the bacterial periplasmic membrane.

As indicated above, it will typically be desired in accordance with theinvention to provide a ligand which has been labeled with one or moredetectable agent(s). This can be carried out, for example, by linkingthe ligand to at least one detectable agent to form a conjugate. Forexample, it is conventional to link or covalently bind or complex atleast one detectable molecule or moiety. A “label” or “detectable label”is a compound and/or element that can be detected due to specificfunctional properties, and/or chemical characteristics, the use of whichallows the ligand to which it is attached to be detected, and/or furtherquantified if desired. Examples of labels which could be used with theinvention include, but are not limited to, enzymes, radiolabels,haptens, fluorescent labels, phosphorescent molecules, chemiluminescentmolecules, chromophores, luminescent molecules, photoaffinity molecules,colored particles or ligands, such as biotin.

In one embodiment of the invention, a visually-detectable marker is usedsuch that automated screening of cells for the label can be carried out.In particular, fluorescent labels are beneficial in that they allow useof flow cytometry for isolation of cells expressing a desired bindingprotein or antibody. Examples of agents that may be detected byvisualization with an appropriate instrument are known in the art, asare methods for their attachment to a desired ligand (see, e.g., U.S.Pat. Nos. 5,021,236; 4,938,948; and 4,472,509, each incorporated hereinby reference). Such agents can include paramagnetic ions; radioactiveisotopes; fluorochromes; NMR-detectable substances and substances forX-ray imaging. Types of fluorescent labels that may be used with theinvention will be well known to those of skill in the art and include,for example, Alexa 350, Alexa 430, AMCA, BODIPY 630/650, BODIPY 650/665,BODIPY-FL, BODIPY-R6G, BODIPY-TMR, BODIPY-TRX, Cascade Blue, Cy3,Cy5,6-FAM, Fluorescein Isothiocyanate, HEX, 6-JOE, Oregon Green 488,Oregon Green 500, Oregon Green 514, Pacific Blue, REG, Rhodamine Green,Rhodamine Red, Renographin, ROX, TAMRA, TET, Tetramethylrhodamine,and/or Texas Red.

Magnetic screening techniques are well known to those of skill in theart (see, for example, U.S. Pat. Nos. 4,988,618, 5,567,326 and5,779,907). Examples of paramagnetic ions that could be used as labelsin accordance with such techniques include ions such as chromium (III),manganese (II), iron (III), iron (II), cobalt (II), nickel (II), copper(II), neodymium (III), samarium (III), ytterbium (III), gadolinium(III), vanadium (II), terbium (III), dysprosium (III), holmium (III)and/or erbium (III). Ions useful in other contexts include but are notlimited to lanthanum (III), gold (III), lead (II), and especiallybismuth (III).

Another type of ligand conjugate contemplated in the present inventionare those where the ligand is linked to a secondary binding moleculeand/or to an enzyme (an enzyme tag) that will generate a colored productupon contact with a chromogenic substrate. Examples of such enzymesinclude urease, alkaline phosphatase, (horseradish) hydrogen peroxidaseor glucose oxidase. In such instances, it will be desired that cellsselected remain viable. Preferred secondary binding ligands are biotinand/or avidin and streptavidin compounds. The use of such labels is wellknown to those of skill in the art and are described, for example, inU.S. Pat. Nos. 3,817,837; 3,850,752; 3,939,350; 3,996,345; 4,277,437;4,275,149 and 4,366,241; each incorporated herein by reference.

Molecules containing azido groups also may be used to form covalentbonds to proteins through reactive nitrene intermediates that aregenerated by low intensity ultraviolet light (Potter & Haley, 1983). Inparticular, 2- and 8-azido analogues of purine nucleotides have beenused as site-directed photoprobes to identify nucleotide-bindingproteins in crude cell extracts (Owens & Haley, 1987; Atherton et al.,1985). The 2- and 8-azido nucleotides have also been used to mapnucleotide-binding domains of purified proteins (Khatoon et al., 1989;King et al., 1989; and Dholakia et al., 1989) and may be used as ligandbinding agents.

Labeling can be carried out by any of the techniques well known to thoseof skill in the art. For instance, ligands can be labeled by contactingthe ligand with the desired label and a chemical oxidizing agent such assodium hypochlorite, or an enzymatic oxidizing agent, such aslactoperoxidase. Similarly, a ligand exchange process could be used.Alternatively, direct labeling techniques may be used, e.g., byincubating the label, a reducing agent such as SNCl₂, a buffer solutionsuch as sodium-potassium phthalate solution, and the ligand.Intermediary functional groups on the ligand could also be used, forexample, to bind labels to a ligand in the presence ofdiethylenetriaminepentaacetic acid (DTPA) or ethylene diaminetetraceticacid (EDTA).

Other methods are also known in the art for the attachment orconjugation of a ligand to its conjugate moiety. Some attachment methodsinvolve the use of an organic chelating agent such asdiethylenetriaminepentaacetic acid anhydride (DTPA);ethylenetriaminetetraacetic acid; N-chloro-p-toluenesulfonamide; and/ortetrachloro-3α-6α-diphenylglycouril-3 attached to the ligand (U.S. Pat.Nos. 4,472,509 and 4,938,948, each incorporated herein by reference).Ligands also may be reacted with an enzyme in the presence of a couplingagent such as glutaraldehyde or periodate. Conjugates with fluoresceinmarkers can be prepared in the presence of these coupling agents or byreaction with an isothiocyanate. In U.S. Pat. No. 4,938,948, imaging ofbreast tumors is achieved using monoclonal antibodies and the detectableimaging moieties are bound to the antibody using linkers such asmethyl-p-hydroxybenzimidate orN-succinimidyl-3-(4-hydroxyphenyl)propionate.

The ability to specifically label periplasmic expressed proteins withappropriate fluorescent ligands also has applications other than libraryscreening. Specifically labeling with fluorescent ligands and flowcytometry can be used for monitoring production during proteinmanufacturing. While flow cytometry has been used previously for theanalysis of bacterial cells, it has not been used for the specificlabeling and quantitation of periplasmic proteins. However, a largenumber of commercially important proteins including IGF-1, severalinterleukins, enzymes such as urokinase-type plasminogen activator,antibody fragments, inhibitors (e.g., bovine pancreatic trypsininhibitor) are expressed in recombinant bacteria in a form secreted intothe periplasmic space. The level of production of such proteins withineach cell in a culture can be monitored by utilizing an appropriatefluorescent ligand and flow cytometric analysis, according to thetechniques taught by the present invention.

Generally, monitoring protein expression requires cell lysis anddetection of the protein by immunological techniques or followingchromatographic separation. However, ELISA or western blot analysis istime-consuming and does not provide information on the distribution ofexpression among a cell population and cannot be used for on-linemonitoring (Thorstenson et al., 1997; Berrier et al., 2000). Incontrast, FACS labeling is rapid and simple and can well be applied toonline monitoring of industrial size fermentations of recombinantproteins expressed in Gram-negative bacteria. Similarly, the inventioncould be used to monitor the production of a particular byproduct of abiological reaction. This also could be used to measure the relativeconcentration or specific activity of an enzyme expressed in vivo in abacterium or provided ex vivo.

Once a ligand-binding protein, such as an antibody, has been isolated inaccordance with the invention, it may be desired to link the molecule toat least one agent to form a conjugate to enhance the utility of thatmolecule. For example, in order to increase the efficacy of antibodymolecules as diagnostic or therapeutic agents, it is conventional tolink or covalently bind or complex at least one desired molecule ormoiety. Such a molecule or moiety may be, but is not limited to, atleast one effector or reporter molecule. Effector molecules comprisemolecules having a desired activity, e.g., cytotoxic activity.Non-limiting examples of effector molecules which have been attached toantibodies include toxins, anti-tumor agents, therapeutic enzymes,radio-labeled nucleotides, antiviral agents, chelating agents,cytokines, growth factors, and oligo- or poly-nucleotides. By contrast,a reporter molecule is defined as any moiety which may be detected usingan assay. Techniques for labeling such a molecule are known to those ofskill in the art and have been described herein above.

Labeled binding proteins such as antibodies which have been prepared inaccordance with the invention may also then be employed, for example, inimmunodetection methods for binding, purifying, removing, quantifyingand/or otherwise generally detecting biological components such asprotein(s), polypeptide(s) or peptide(s). Some immunodetection methodsinclude enzyme linked immunosorbent assay (ELISA), radioimmunoassay(RIA), immunoradiometric assay, fluoroimmunoassay, chemiluminescentassay, bioluminescent assay, and Western blot to mention a few. Thesteps of various useful immunodetection methods have been described inthe scientific literature, such as, e.g., Doolittle M H and Ben-Zeev O,1999; Gulbis B and Galand P, 1993; and De Jager R et al., 1993, eachincorporated herein by reference. Such techniques include binding assayssuch as the various types of enzyme linked immunosorbent assays (ELISAs)and/or radioimmunoassays (RIA) known in the art.

The ligand-binding molecules, including antibodies, prepared inaccordance with the present invention may also, for example, inconjunction with both fresh-frozen and/or formalin-fixed,paraffin-embedded tissue blocks prepared for study byimmunohistochemistry (IHC). The method of preparing tissue blocks fromthese particulate specimens has been successfully used in previous IHCstudies of various prognostic factors, and/or is well known to those ofskill in the art (Abbondanzo et al., 1990).

VI. Automated Screening with Flow Cytometry

In one embodiment of the invention, fluorescence activated cell sorting(FACS) screening or other automated flow cytometric techniques may beused for the efficient isolation of a bacterial cell comprising alabeled ligand bound to a candidate molecule and linked to the outerface of the cytoplasmic membrane of the bacteria. Instruments forcarrying out flow cytometry are known to those of skill in the art andare commercially available to the public. Examples of such instrumentsinclude FACS Star Plus, FACScan and FACSort instruments from BectonDickinson (Foster City, Calif.) Epics C from Coulter Epics Division(Hialeah, Fla.) and MOFLO™ from Cytomation (Colorado Springs, Co).

Flow cytometric techniques in general involve the separation of cells orother particles in a liquid sample. Typically, the purpose of flowcytometry is to analyze the separated particles for one or morecharacteristics thereof, for example, presence of a labeled ligand orother molecule. The basis steps of flow cytometry involve the directionof a fluid sample through an apparatus such that a liquid stream passesthrough a sensing region. The particles should pass one at a time by thesensor and are categorized base on size, refraction, light scattering,opacity, roughness, shape, fluorescence, etc.

Rapid quantitative analysis of cells proves useful in biomedicalresearch and medicine. Apparati permit quantitative multiparameteranalysis of cellular properties at rates of several thousand cells persecond. These instruments provide the ability to differentiate amongcell types. Data are often displayed in one-dimensional (histogram) ortwo-dimensional (contour plot, scatter plot) frequency distributions ofmeasured variables. The partitioning of multiparameter data filesinvolves consecutive use of the interactive one- or two-dimensionalgraphics programs.

Quantitative analysis of multiparameter flow cytometric data for rapidcell detection consists of two stages: cell class characterization andsample processing. In general, the process of cell classcharacterization partitions the cell feature into cells of interest andnot of interest. Then, in sample processing, each cell is classified inone of the two categories according to the region in which it falls.Analysis of the class of cells is very important, as high detectionperformance may be expected only if an appropriate characteristic of thecells is obtained.

Not only is cell analysis performed by flow cytometry, but so too issorting of cells. In U.S. Pat. No. 3,826,364, an apparatus is disclosedwhich physically separates particles, such as functionally differentcell types. In this machine, a laser provides illumination which isfocused on the stream of particles by a suitable lens or lens system sothat there is highly localized scatter from the particles therein. Inaddition, high intensity source illumination is directed onto the streamof particles for the excitation of fluorescent particles in the stream.Certain particles in the stream may be selectively charged and thenseparated by deflecting them into designated receptacles. A classic formof this separation is via fluorescent-tagged antibodies, which are usedto mark one or more cell types for separation.

Other examples of methods for flow cytometry that could include, but arenot limited to, those described in U.S. Pat. Nos. 4,284,412; 4,989,977;4,498,766; 5,478,722; 4,857,451; 4,774,189; 4,767,206; 4,714,682;5,160,974; and 4,661,913, each of the disclosures of which arespecifically incorporated herein by reference.

For the present invention, an important aspect of flow cytometry is thatmultiple rounds of screening can be carried out sequentially. Cells maybe isolated from an initial round of sorting and immediatelyreintroduced into the flow cytometer and screened again to improve thestringency of the screen. Another advantage known to those of skill inthe art is that nonviable cells can be recovered using flow cytometry.Since flow cytometry is essentially a particle sorting technology, theability of a cell to grow or propagate is not necessary. Techniques forthe recovery of nucleic acids from such non-viable cells are well knownin the art and may include, for example, use of template-dependentamplification techniques including PCR.

VII. Nucleic Acid-Based Expression Systems

Nucleic acid-based expression systems may find use, in certainembodiments of the invention, for the expression of recombinantproteins. For example, one embodiment of the invention involvestransformation of Gram negative bacteria with the coding sequences offusion polypeptides comprising a candidate antibody or other bindingprotein having affinity for a selected ligand and the expression of suchmolecules on the cytoplasmic membrane of the Gram negative bacteria. Inother embodiments of the invention, expression of such coding sequencesmay be carried, for example, in eukaryotic host cells for thepreparation of isolated binding proteins having specificity for thetarget ligand. The isolated protein could then be used in one or moretherapeutic or diagnostic applications.

A. Methods of Nucleic Acid Delivery

Certain aspects of the invention may comprise delivery of nucleic acidsto target cells. For example, bacterial host cells may be transformedwith nucleic acids encoding candidate molecules potentially capablebinding a target ligand, In particular embodiments of the invention, itmay be desired to target the expression to the cytoplasmic membrane ofthe bacteria. Transformation of eukaryotic host cells may similarly finduse in the expression of various candidate molecules identified ascapable of binding a target ligand.

Suitable methods for nucleic acid delivery for transformation of a cellare believed to include virtually any method by which a nucleic acid(e.g., DNA) can be introduced into such a cell, or even an organellethereof. Such methods include, but are not limited to, direct deliveryof DNA such as by injection (U.S. Pat. Nos. 5,994,624, 5,981,274,5,945,100, 5,780,448, 5,736,524, 5,702,932, 5,656,610, 5,589,466 and5,580,859, each incorporated herein by reference), includingmicroinjection (Harlan and Weintraub, 1985; U.S. Pat. No. 5,789,215,incorporated herein by reference); by electroporation (U.S. Pat. No.5,384,253, incorporated herein by reference); by calcium phosphateprecipitation (Graham and Van Der Eb, 1973; Chen and Okayama, 1987;Rippe et al., 1990); by using DEAE-dextran followed by polyethyleneglycol (Gopal, 1985); by direct sonic loading (Fechheimer et al., 1987);by liposome mediated transfection (Nicolau and Sene, 1982; Fraley etal., 1979; Nicolau et al., 1987; Wong et al., 1980; Kaneda et al., 1989;Kato et al., 1991); by microprojectile bombardment (PCT Application Nos.WO 94/09699 and 95/06128; U.S. Pat. Nos. 5,610,042; 5,322,783 5,563,055,5,550,318, 5,538,877 and 5,538,880, and each incorporated herein byreference); by agitation with silicon carbide fibers (Kaeppler et al.,1990; U.S. Pat. Nos. 5,302,523 and 5,464,765, each incorporated hereinby reference); by Agrobacterium-mediated transformation (U.S. Pat. Nos.5,591,616 and 5,563,055, each incorporated herein by reference); or byPEG-mediated transformation of protoplasts (Omirulleh et al., 1993; U.S.Pat. Nos. 4,684,611 and 4,952,500, each incorporated herein byreference); by desiccation/inhibition-mediated DNA uptake (Potrykus etal., 1985). Through the application of techniques such as these,organelle(s), cell(s), tissue(s) or organism(s) may be stably ortransiently transformed.

1. Electroporation

In certain embodiments of the present invention, a nucleic acid isintroduced into a cell via electroporation. Electroporation involves theexposure of a suspension of cells and DNA to a high-voltage electricdischarge. In some variants of this method, certain cell wall-degradingenzymes, such as pectin-degrading enzymes, are employed to render thetarget recipient cells more susceptible to transformation byelectroporation than untreated cells (U.S. Pat. No. 5,384,253,incorporated herein by reference). Alternatively, recipient cells can bemade more susceptible to transformation by mechanical wounding.

2. Calcium Phosphate

In other embodiments of the present invention, a nucleic acid isintroduced to the cells using calcium phosphate precipitation. Human KBcells have been transfected with adenovirus 5 DNA (Graham and Van DerEb, 1973) using this technique. Also in this manner, mouse L(A9), mouseC127, CHO, CV-1, BHK, NIH3T3 and HeLa cells were transfected with aneomycin marker gene (Chen and Okayama, 1987), and rat hepatocytes weretransfected with a variety of marker genes (Rippe et al., 1990).

B. Vectors

Vectors may find use with the current invention, for example, in thetransformation of a Gram negative bacterium with a nucleic acid sequenceencoding a candidate polypeptide which one wishes to screen for abilityto bind a target ligand. In one embodiment of the invention, an entireheterogeneous “library” of nucleic acid sequences encoding targetpolypeptides may be introduced into a population of bacteria, therebyallowing screening of the entire library. The term “vector” is used torefer to a carrier nucleic acid molecule into which a nucleic acidsequence can be inserted for introduction into a cell where it can bereplicated. A nucleic acid sequence can be “exogenous,” or“heterologous”, which means that it is foreign to the cell into whichthe vector is being introduced or that the sequence is homologous to asequence in the cell but in a position within the host cell nucleic acidin which the sequence is ordinarily not found. Vectors include plasmids,cosmids, viruses (bacteriophage, animal viruses, and plant viruses), andartificial chromosomes (e.g., YACs). One of skill in the art mayconstruct a vector through standard recombinant techniques, which aredescribed in Maniatis et al., 1988 and Ausubel et al., 1994, both ofwhich references are incorporated herein by reference.

The term “expression vector” refers to a vector containing a nucleicacid sequence coding for at least part of a gene product capable ofbeing transcribed. In some cases, RNA molecules are then translated intoa protein, polypeptide, or peptide. In other cases, these sequences arenot translated, for example, in the production of antisense molecules orribozymes. Expression vectors can contain a variety of “controlsequences,” which refer to nucleic acid sequences necessary for thetranscription and possibly translation of an operably linked codingsequence in a particular host organism. In addition to control sequencesthat govern transcription and translation, vectors and expressionvectors may contain nucleic acid sequences that serve other functions aswell and are described infra.

1. Promoters and Enhancers

A “promoter” is a control sequence that is a region of a nucleic acidsequence at which initiation and rate of transcription are controlled.It may contain genetic elements at which regulatory proteins andmolecules may bind such as RNA polymerase and other transcriptionfactors. The phrases “operatively positioned,” “operatively linked,”“under control,” and “under transcriptional control” mean that apromoter is in a correct functional location and/or orientation inrelation to a nucleic acid sequence to control transcriptionalinitiation and/or expression of that sequence. A promoter may or may notbe used in conjunction with an “enhancer,” which refers to a cis-actingregulatory sequence involved in the transcriptional activation of anucleic acid sequence.

A promoter may be one naturally associated with a gene or sequence, asmay be obtained by isolating the 5′ non-coding sequences locatedupstream of the coding segment and/or exon. Such a promoter can bereferred to as “endogenous.” Similarly, an enhancer may be one naturallyassociated with a nucleic acid sequence, located either downstream orupstream of that sequence. Alternatively, certain advantages will begained by positioning the coding nucleic acid segment under the controlof a recombinant or heterologous promoter, which refers to a promoterthat is not normally associated with a nucleic acid sequence in itsnatural environment. A recombinant or heterologous enhancer refers alsoto an enhancer not normally associated with a nucleic acid sequence inits natural environment. Such promoters or enhancers may includepromoters or enhancers of other genes, and promoters or enhancersisolated from any other prokaryotic, viral, or eukaryotic cell, andpromoters or enhancers not “naturally occurring,” i.e., containingdifferent elements of different transcriptional regulatory regions,and/or mutations that alter expression. In addition to producing nucleicacid sequences of promoters and enhancers synthetically, sequences maybe produced using recombinant cloning and/or nucleic acid amplificationtechnology, including PCR™, in connection with the compositionsdisclosed herein (see U.S. Pat. Nos. 4,683,202, 5,928,906, eachincorporated herein by reference). Furthermore, it is contemplated thatthe control sequences that direct transcription and/or expression ofsequences within non-nuclear organelles such as mitochondria,chloroplasts, and the like, can be employed as well.

Naturally, it will be important to employ a promoter and/or enhancerthat effectively directs the expression of the DNA segment in the celltype, organelle, and organism chosen for expression. One example of suchpromoter that may be used with the invention is the E. coli arabinosepromoter. Those of skill in the art of molecular biology generally arefamiliar with the use of promoters, enhancers, and cell typecombinations for protein expression, for example, see Sambrook et al.(1989), incorporated herein by reference. The promoters employed may beconstitutive, tissue-specific, inducible, and/or useful under theappropriate conditions to direct high level expression of the introducedDNA segment, such as is advantageous in the large-scale production ofrecombinant proteins and/or peptides. The promoter may be heterologousor endogenous.

2. Initiation Signals and Internal Ribosome Binding Sites

A specific initiation signal also may be required for efficienttranslation of coding sequences. These signals include the ATGinitiation codon or adjacent sequences. Exogenous translational controlsignals, including the ATG initiation codon, may need to be provided.One of ordinary skill in the art would readily be capable of determiningthis and providing the necessary signals. It is well known that theinitiation codon must be “in-frame” with the reading frame of thedesired coding sequence to ensure translation of the entire insert. Theexogenous translational control signals and initiation codons can beeither natural or synthetic. The efficiency of expression may beenhanced by the inclusion of appropriate transcription enhancerelements.

3. Multiple Cloning Sites

Vectors can include a multiple cloning site (MCS), which is a nucleicacid region that contains multiple restriction enzyme sites, any ofwhich can be used in conjunction with standard recombinant technology todigest the vector (see Carbonelli et al., 1999, Levenson et al., 1998,and Cocea, 1997, incorporated herein by reference.) “Restriction enzymedigestion” refers to catalytic cleavage of a nucleic acid molecule withan enzyme that functions only at specific locations in a nucleic acidmolecule. Many of these restriction enzymes are commercially available.Use of such enzymes is understood by those of skill in the art.Frequently, a vector is linearized or fragmented using a restrictionenzyme that cuts within the MCS to enable exogenous sequences to beligated to the vector. “Ligation” refers to the process of formingphosphodiester bonds between two nucleic acid fragments, which may ormay not be contiguous with each other. Techniques involving restrictionenzymes and ligation reactions are well known to those of skill in theart of recombinant technology.

4. Termination Signals

The vectors or constructs prepared in accordance with the presentinvention will generally comprise at least one termination signal. A“termination signal” or “terminator” is comprised of the DNA sequencesinvolved in specific termination of an RNA transcript by an RNApolymerase. Thus, in certain embodiments, a termination signal that endsthe production of an RNA transcript is contemplated. A terminator may benecessary in vivo to achieve desirable message levels.

Terminators contemplated for use in the invention include any knownterminator of transcription described herein or known to one of ordinaryskill in the art, including but not limited to, for example, rhpdependent or rho independent terminators. In certain embodiments, thetermination signal may be a lack of transcribable or translatablesequence, such as due to a sequence truncation.

5. Origins of Replication

In order to propagate a vector in a host cell, it may contain one ormore origins of replication sites (often termed “ori”), which is aspecific nucleic acid sequence at which replication is initiated.Alternatively an autonomously replicating sequence (ARS) can be employedif the host cell is yeast.

6. Selectable and Screenable Markers

In certain embodiments of the invention, cells containing a nucleic acidconstruct of the present invention may be identified in vitro or in vivoby including a marker in the expression vector. Such markers wouldconfer an identifiable change to the cell permitting easy identificationof cells containing the expression vector. Generally, a selectablemarker is one that confers a property that allows for selection. Apositive selectable marker is one in which the presence of the markerallows for its selection, while a negative selectable marker is one inwhich its presence prevents its selection. An example of a positiveselectable marker is a drug resistance marker.

Usually the inclusion of a drug selection marker aids in the cloning andidentification of transformants, for example, genes that conferresistance to neomycin, puromycin, hygromycin, DHFR, GPT, zeocin andhistidinol are useful selectable markers. In addition to markersconferring a phenotype that allows for the discrimination oftransformants based on the implementation of conditions, other types ofmarkers including screenable markers such as GFP, whose basis iscalorimetric analysis, are also contemplated. Alternatively, screenableenzymes such as herpes simplex virus thymidine kinase (tk) orchloramphenicol acetyltransferase (CAT) may be utilized. One of skill inthe art would also know how to employ immunologic markers, possibly inconjunction with FACS analysis. The marker used is not believed to beimportant, so long as it is capable of being expressed simultaneouslywith the nucleic acid encoding a gene product. Further examples ofselectable and screenable markers are well known to one of skill in theart.

C. Host Cells

As used herein, the terms “cell,” “cell line,” and “cell culture” may beused interchangeably. All of these terms also include their progeny,which are any and all subsequent generations. It is understood that allprogeny may not be identical due to deliberate or inadvertent mutations.In the context of expressing a heterologous nucleic acid sequence, “hostcell” refers to a prokaryotic cell, and it includes any transformableorganism that is capable of replicating a vector and/or expressing aheterologous gene encoded by a vector. A host cell can, and has been,used as a recipient for vectors. A host cell may be “transfected” or“transformed,” which refers to a process by which exogenous nucleic acidis transferred or introduced into the host cell. A transformed cellincludes the primary subject cell and its progeny.

In particular embodiments of the invention, a host cell is a Gramnegative bacterial cell. These bacteria are suited for use with theinvention in that they posses a periplasmic space between the inner andouter membrane and, particularly, the aforementioned inner membranebetween the periplasm and cytoplasm, which is also known as thecytoplasmic membrane. As such, any other cell with such a periplasmicspace could be used in accordance with the invention. Examples of Gramnegative bacteria that may find use with the invention may include, butare not limited to, E. coli, Pseudomonas aeruginosa, Vibrio cholera,Salmonella typhimurium, Shigella flexneri, Haemophilus influenza,Bordotella pertussi, Erwinia amylovora, Rhizobium sp. The Gram negativebacterial cell may be still further defined as bacterial cell which hasbeen transformed with the coding sequence of a fusion polypeptidecomprising a candidate binding polypeptide capable of binding a selectedligand. The polypeptide is anchored to the outer face of the cytoplasmicmembrane, facing the periplasmic space, and may comprise an antibodycoding sequence or another sequence. One means for expression of thepolypeptide is by attaching a leader sequence to the polypeptide capableof causing such directing.

Numerous prokaryotic cell lines and cultures are available for use as ahost cell, and they can be obtained through the American Type CultureCollection (ATCC), which is an organization that serves as an archivefor living cultures and genetic materials (world wide web at atcc.org).An appropriate host can be determined by one of skill in the art basedon the vector backbone and the desired result. A plasmid or cosmid, forexample, can be introduced into a prokaryote host cell for replicationof many vectors. Bacterial cells used as host cells for vectorreplication and/or expression include DH5α, JM109, and KC8, as well as anumber of commercially available bacterial hosts such as SURE® CompetentCells and SOLOPACK™ Gold Cells (STRATAGENE®, La Jolla). Alternatively,bacterial cells such as E. coliLE392 could be used as host cells forbacteriophage.

Many host cells from various cell types and organisms are available andwould be known to one of skill in the art. Similarly, a viral vector maybe used in conjunction with a prokaryotic host cell, particularly onethat is permissive for replication or expression of the vector. Somevectors may employ control sequences that allow it to be replicatedand/or expressed in both prokaryotic and eukaryotic cells. One of skillin the art would further understand the conditions under which toincubate all of the above described host cells to maintain them and topermit replication of a vector. Also understood and known are techniquesand conditions that would allow large-scale production of vectors, aswell as production of the nucleic acids encoded by vectors and theircognate polypeptides, proteins, or peptides.

D. Expression Systems

Numerous expression systems exist that comprise at least a part or allof the compositions discussed above. Such systems could be used, forexample, for the production of a polypeptide product identified inaccordance with the invention as capable of binding a particular ligand.Prokaryote-based systems can be employed for use with the presentinvention to produce nucleic acid sequences, or their cognatepolypeptides, proteins and peptides. Many such systems are commerciallyand widely available. Other examples of expression systems comprise ofvectors containing a strong prokaryotic promoter such as T7, Tac, Trc,BAD, lambda pL, Tetracycline or Lac promoters, the pET Expression Systemand an E. coli expression system.

E. Candidate Binding Proteins and Antibodies

In certain aspects of the invention, antibodies are expressed on thecytoplasmic or in the periplasmic space membrane of a host bacterialcell. By expression of a heterogeneous population of such antibodies,those antibodies having a high affinity for a target ligand may beidentified. The identified antibodies may then be used in variousdiagnostic or therapeutic applications, as described herein.

As used herein, the term “antibody” is intended to refer broadly to anyimmunologic binding agent such as IgG, IgM, IgA, IgD and IgE. The term“antibody” is also used to refer to any antibody-like molecule that hasan antigen binding region, and includes antibody fragments such as Fab′,Fab, F(ab′)₂, single domain antibodies (DABs), Fv, scFv (single chainFv), and engineering multivalent antibody fragments such as dibodies,tribodies and multibodies. The techniques for preparing and usingvarious antibody-based constructs and fragments are well known in theart. Means for preparing and characterizing antibodies are also wellknown in the art (See, e.g., Antibodies: A Laboratory Manual, ColdSpring Harbor Laboratory, 1988; incorporated herein by reference).However, in preferred aspects, an antibody for use in the invention isan intact antibody comprising a heavy chain constant domain.

Once an antibody having affinity for a target ligand is identified, theantibody or ligand-binding polypeptide may be purified, if desired,using filtration, centrifugation and various chromatographic methodssuch as HPLC or affinity chromatography. Fragments of such polypeptides,including antibodies, can be obtained from the antibodies so produced bymethods which include digestion with enzymes, such as pepsin or papain,and/or by cleavage of disulfide bonds by chemical reduction.Alternatively, antibody or other polypeptides, including proteinfragments, encompassed by the present invention can be synthesized usingan automated peptide synthesizer.

A molecular cloning approach comprises one suitable method for thegeneration of a heterogeneous population of candidate antibodies thatmay then be screened in accordance with the invention for affinity totarget ligands. In one embodiment of the invention, combinatorialimmunoglobulin phagemid can be prepared from RNA isolated from thespleen of an animal. By immunizing an animal with the ligand to bescreened, the assay may be targeted to the particular antigen. Theadvantages of this approach over conventional techniques are thatapproximately 10⁴ times as many antibodies can be produced and screenedin a single round, and that new specificities are generated by H and Lchain combination which further increases the chance of findingappropriate antibodies.

VIII. Manipulation and Detection of Nucleic Acids

In certain embodiments of the invention, it may be desired to employ oneor more techniques for the manipulation, isolation and/or detection ofnucleic acids. Such techniques may include, for example, the preparationof vectors for transformation of host cells as well as methods forcloning selected nucleic acid segments from a transgenic cell.Methodology for carrying out such manipulations will be well known tothose of skill in the art in light of the instant disclosure.

Nucleic acids used as a template for amplification may be isolated fromcells, tissues or other samples according to standard methodologies(Sambrook et al., 1989). In certain embodiments, analysis may beperformed on whole cell or tissue homogenates or biological fluidsamples without substantial purification of the template nucleic acid.The nucleic acid may be genomic DNA or fractionated or whole cell RNA.Where RNA is used, it may be desired to first convert the RNA to acomplementary DNA.

The term “primer,” as used herein, is meant to encompass any nucleicacid that is capable of priming the synthesis of a nascent nucleic acidin a template-dependent process. Typically, primers are oligonucleotidesfrom ten to twenty and/or thirty base pairs in length, but longersequences can be employed. Primers may be provided in double-strandedand/or single-stranded form, although the single-stranded form ispreferred.

Pairs of primers designed to selectively hybridize to nucleic acidscorresponding to a selected nucleic acid sequence are contacted with thetemplate nucleic acid under conditions that permit selectivehybridization. Depending upon the desired application, high stringencyhybridization conditions may be selected that will only allowhybridization to sequences that are completely complementary to theprimers. In other embodiments, hybridization may occur under reducedstringency to allow for amplification of nucleic acids comprising one ormore mismatches with the primer sequences. Once hybridized, thetemplate-primer complex is contacted with one or more enzymes thatfacilitate template-dependent nucleic acid synthesis. Multiple rounds ofamplification, also referred to as “cycles,” are conducted until asufficient amount of amplification product is produced.

The amplification product may be detected or quantified. In certainapplications, the detection may be performed by visual means.Alternatively, the detection may involve indirect identification of theproduct via chemiluminescence, radioactive scintigraphy of incorporatedradiolabel or fluorescent label or even via a system using electricaland/or thermal impulse signals (Affymax technology; Bellus, 1994).

A number of template dependent processes are available to amplify theoligonucleotide sequences present in a given template sample. One of thebest known amplification methods is the polymerase chain reaction(referred to as PCR™) which is described in detail in U.S. Pat. Nos.4,683,195, 4,683,202 and 4,800,159, and in Innis et al., 1988, each ofwhich is incorporated herein by reference in their entirety.

A reverse transcriptase PCR™ amplification procedure may be performed toquantify the amount of mRNA amplified. Methods of reverse transcribingRNA into cDNA are well known (see Sambrook et al., 1989). Alternativemethods for reverse transcription utilize thermostable DNA polymerases.These methods are described in WO 90/07641. Polymerase chain reactionmethodologies are well known in the art. Representative methods ofRT-PCR are described in U.S. Pat. No. 5,882,864.

Another method for amplification is ligase chain reaction (“LCR”),disclosed in European Application 320 308, incorporated herein byreference in its entirety. U.S. Pat. No. 4,883,750 describes a methodsimilar to LCR for binding probe pairs to a target sequence. A methodbased on PCR™ and oligonucleotide ligase assay (OLA), disclosed in U.S.Pat. No. 5,912,148, may also be used.

Alternative methods for amplification of target nucleic acid sequencesthat may be used in the practice of the present invention are disclosedin U.S. Pat. Nos. 5,843,650, 5,846,709, 5,846,783, 5,849,546, 5,849,497,5,849,547, 5,858,652, 5,866,366, 5,916,776, 5,922,574, 5,928,905,5,928,906, 5,932,451, 5,935,825, 5,939,291 and 5,942,391, GB ApplicationNo. 2 202 328, and in PCT Application No. PCT/US89/01025, each of whichis incorporated herein by reference in its entirety.

Qbeta Replicase, described in PCT Application No. PCT/US87/00880, mayalso be used as an amplification method in the present invention. Inthis method, a replicative sequence of RNA that has a regioncomplementary to that of a target is added to a sample in the presenceof an RNA polymerase. The polymerase will copy the replicative sequencewhich may then be detected.

An isothermal amplification method, in which restriction endonucleasesand ligases are used to achieve the amplification of target moleculesthat contain nucleotide 5′-[alpha-thio]-triphosphates in one strand of arestriction site may also be useful in the amplification of nucleicacids in the present invention (Walker et al., 1992). StrandDisplacement Amplification (SDA), disclosed in U.S. Pat. No. 5,916,779,is another method of carrying out isothermal amplification of nucleicacids which involves multiple rounds of strand displacement andsynthesis, i.e., nick translation.

Other nucleic acid amplification procedures include transcription-basedamplification systems (TAS), including nucleic acid sequence basedamplification (NASBA) and 3SR (Kwoh et al., 1989; Gingeras et al., PCTApplication WO 88/10315, incorporated herein by reference in theirentirety). European Application No. 329 822 disclose a nucleic acidamplification process involving cyclically synthesizing single-strandedRNA (“ssRNA”), ssDNA, and double-stranded DNA (dsDNA), which may be usedin accordance with the present invention.

PCT Application WO 89/06700 (incorporated herein by reference in itsentirety) discloses a nucleic acid sequence amplification scheme basedon the hybridization of a promoter region/primer sequence to a targetsingle-stranded DNA (“ssDNA”) followed by transcription of many RNAcopies of the sequence. This scheme is not cyclic, i.e., new templatesare not produced from the resultant RNA transcripts. Other amplificationmethods include “race” and “one-sided PCR” (Frohman, 1990; Ohara et al.,1989).

IX. EXAMPLES

The following examples are included to demonstrate preferred embodimentsof the invention. It should be appreciated by those of skill in the artthat the techniques disclosed in the examples which follow representtechniques discovered by the inventor to function well in the practiceof the invention, and thus can be considered to constitute preferredmodes for its practice. However, those of skill in the art should, inlight of the present disclosure, appreciate that many changes can bemade in the specific embodiments which are disclosed and still obtain alike or similar result without departing from the spirit and scope ofthe invention.

Example 1 Construction of Vectors for Dicistronic Expression of IntactIgG Antibodies in E. coli Periplasm and for Fluorescence Detection ofNlpA-ZZ-IgG Displaying Cells

The pMAZ360-IgG expression vector was designed to allow efficientexpression of soluble full-assembled IgG antibodies in E. coliperiplasm. The plasmid facilitates convenient cloning of VH and Vκ geneslinked to human γ1 and κ constant domains, respectively as a dicistronicoperon downstream to a lac promoter (FIG. 2). Briefly, the first cistroncomprised of the light chain fused to a pelB leader sequence forsecretion into the periplasm. Two stop codons were engineered at the endof the light chain cistron, followed by approximately 20 nucleotidesbefore the Shine-Dalgarno sequence of the next cistron. A second pelBleader sequence was incorporated to the N-terminus of the heavy chainfor efficient secretion into the periplasm. VH domains are introducedinto the vector via NheI/HindIII restriction sites, whereas, VL domainsare cloned as NcoI/NotI restriction fragments. The plasmid contains anampicillin selectable marker and the packaging signal of f1 that enablesthe packaging of the plasmid as ssDNA in the presence of a helper phage.

The variable domains of a humanized derivative of the anti-Bacillusanthracis Protective Antigen of M18.1 Hum (Harvey et al., 2004) wereintroduced into pMAZ360-IgG expression vector to construct plasmidpMAZ360-M18.1-Hum-IgG (SEQ ID NO:1). In the same way the murineanti-Digoxin 26.10 (Chen et al., 1999) variable domains were cloned intopMAZ360-IgG to construct plasmid pMAZ360-26.10-IgG (SEQ ID NO:2).

For expression of intact IgG antibodies in the bacterial periplasm, E.coli JUDE1 cells transformed with expression plasmidspMAZ360-M18.1-Hum-IgG or pMAZ360-26.10-IgG were grown in a 200 ml scale.Upon induction with IPTG, soluble cell extract fractions of cellsexpressing the M18.1 Hum IgG were tested for full-length IgG productionby Western-blot analysis under non-reducing conditions (FIG. 3A) andreducing conditions (FIG. 3B). Soluble IgG antibodies were purified bypassing the soluble cell extracts on a protein A column. Under theseconditions, M18.1 Hum IgG and 26.10 IgG were purified tonear-homogeneity as based upon separation on 12%/SDS-PAGE undernon-reducing conditions (FIG. 3C) and reducing conditions (FIG. 3D) atyields of about 1 mg/l of cells. As seen by the Western-blot analysis(FIG. 3A) a low degree of intermediate partially assembled IgG productsis also obtained. However the majority of the heavy and light moleculesare presented as tetrameric full-assembled intact IgG.

For capturing soluble expressed intact IgGs via the ZZ moiety fordisplay onto the periplasmic face of the inner-membrane plasmidpBAD33-NlpA-ZZ was constructed (SEQ ID NO:3). In this plasmid the ZZgene was inserted in frame to the C-terminus of a sequence encoding theleader peptide and first six amino acids of the mature NlpA (containingthe putative fatty acylation and inner membrane targeting sites).NlpA-ZZ fusion proteins expressed under the PBAD promoter are secretedto the periplasmic compartment and become tethered to theinner-membrane.

NlpA-ZZ displaying spheroplasts were initially evaluated for theirability to capture externally added IgG antibodies added to theextracellular fluid. Upon induction of protein synthesis with arabinose,the E. coli outer membrane was disrupted by incubation in Tris-EDTA andlysozyme and permeabilized spheroplasts were tested for capturingfluorescently-labeled commercial Human IgG antibodies (FIG. 4A) orbacterially expressed M18.1 Hum IgG antibodies followed by incubationwith FITC-conjugated PA (FIG. 4C). The cell fluorescence was determinedby means of flow cytometry (FC).

Spheroplasted cells displaying the NlpA-ZZ fusion were able to capturethe fluorescently labeled Human IgG antibodies. In control experimentsno labeling was detected with spheroplasts expressing an NlpA fusion toa protein (scFv) that does not bind IgG (FIG. 4B,D). Whole cell ELISAindicated that approximately, 7500 NlpA-ZZ molecules are presented oneach spheroplasted cell. Determination of the equilibrium dissociationconstant (K_(D)) of the complex NlpA-ZZ-IgG by half-maximal bindingrevealed a K_(D) value of less then 100 pM. The K_(D) values obtained inwhole cells are much higher than the equilibrium dissociation constantfor the ZZ:IgG complex measured in solution (estimated as 10 nM, Nilssonet al., 1987; Hober et al., 2006). This difference in affinity is mostlikely a consequence of avidity effects obtained from the high densityof NlpA-ZZ fusion proteins expressed on the periplasmic membrane.

The retention and stability of the NlpA-ZZ-IgG complexes on the surfaceof spheroplasts was determined by monitoring the cells fluorescence(arising from the fluorescently labeled IgG) as a function of time, byflow cytometry (FIG. 5). No reduction in fluorescence signal wasobtained even after three days. Evidently, the high density of NlpA-ZZmolecules anchored on the periplasmic membrane results in high aviditybinding of IgG molecules which in turn gives rise to a very highapparent binding constant for IgG on the spheroplasts.

Example 2 Expression of IgG in Bacteria

A. Construction of Bacterial pMAZ360-IgG Expression Vector and Cloningof Immunoglobulin Genes to be Expressed as Intact IgG in E. coliPeriplasm.

Plasmid pMAZ360-IgG was designed for the efficient expression of solublefully-assembled IgG antibodies in E. coli periplasm as a dicistronicoperon transcribed from a single promoter. Initially, a HindIII site onpMoPac 16 (Hayhurst et al., 2003) was removed by site directedmutagenesis (Strategene, USA. QUICKCHANGE® Mutaganesis Kit) usingprimers HindIII-Mut-FOR and HindIII-Mut-REV (all PCR primers are listedin Table 2). Vectors containing the constant regions for the light andheavy chains of human IgG1 were kindly provided by Nancy Green (AlbertEinstein College of Medicine, USA). The heavy chain gene of M18.1 HumIgG was constructed by PCR. First, the human gamma 1 constant heavychain region (CH1-CH3) was recovered by PCR using primers CH1-FOR andCH3-AscI-REV. In a second PCR reaction the VH region of M18.1 Hum wasamplified using primers M18.1-VH-NheI-FOR and M18.1-VH-HindIII-REV. Theformer introduces a Shine Dalgarno and a silent mutation in the pelBleader incorporating a unique NheI site while the later introduces asequence corresponding to the HindIII site on the N-terminus of the CH1domain. The two PCR products were combined by an overlap extension PCRusing primers M18.1-VH-NheI-FOR and CH3-AscI-REV to give the completeheavy chain gene. The light chain gene of M18.1 Hum IgG was alsoconstructed by PCR. The human Kappa light chain was amplified usingprimers CK-FOR and CK-Stop-REV (Table 2). The later introduces two stopscodons and overlap with the Shine Dalgarno and a pelB leader sequence onthe N-terminus of the VH domain. The VL region of M18.1 Hum wasamplified in a second PCR reaction using primers M18.1-VL-NcoI-FOR andM18.1-VL-Not-REV. The later incorporated a NotI site that correspondswith the NotI sequence on the N-terminus of the Ck domain. The two PCRproducts were combined by an overlap extension PCR using primersM18.1-VL-NcoI-FOR and CK-Stop-REV to give the complete light chain gene.Finally, the heavy and light gene templates were assembled to give acomplete bicistronic IgG gene by overlap extension PCR using primersM18.1-VL-NcoI-FOR and CH3-AscI-REV. The resulting PCR product of ˜2 kbwas purified and digested with restriction enzymes NcoI and AscI andcloned into the pMoPac 16 vector recovered following digestion with thesame enzymes. The resulting vector was named pMAZ360-M18.1-Hum-IgG. VHand VL domains are introduced to the pMAZ360-IgG vector as NheI/HindIIIand NcoI/NotI restriction fragments, respectively. The VH and VLvariable domains of the murine anti-Digoxin 26.10 (Chen et al., 1999)were introduced into plasmid pMAZ360-IgG as NheI/HindIII and SfiI/NotIrestriction fragments, respectively. The resulting plasmid was namedpMAZ360-26.10-IgG.

B. Expression and Purification of Intact IgGs in Bacterial Periplasm

For expression of soluble intact IgG, E. coli JUDE1 cells transformedwith pMAZ360-M18.1 Hum-IgG and pMAZ360-26.10-IgG expression plasmids wasinoculated overnight (ON) at 30° C. in Luria broth (LB) mediumsupplemented with 100 μg/ml ampicillin and 2% (w/v) glucose. Thefollowing day the cultures were diluted into 200 ml of Terrific broth(TB) medium supplemented with 100 μg/ml ampicillin and 2% (w/v) glucoseto give a starting A₆₀₀ of 0.2 and grown at 30° C. When the culturesreached an A₆₀₀ of 1.0, the cells were pelleted for 15 min at 6000 rpmand resuspended in TB medium supplemented with 100 μg/ml ampicillin and1 mM isopropyl-1-thio-D-galactopyranoside (IPTG). Cells were inducedover night at 25° C. for expression of IgG. The following day cells wereharvested and pelleted for 15 min at 6000 rpm. Cell extracts wereprepared by resuspension of the pellet in 20 ml of BUGBUSTER™ HT ProteinExtraction Reagent (NOVAGENE™, USA). Followed rotation head-over-headfor two hours at room temperature (RT) the extracts were clarified bycentrifugation at 12,000 rpm at 4° C. The clarified sample was diluted1:1 with loading buffer (20 mM Na₂HPO₄, 2 Mm NaH₂PO₄) and loaded onto a1-ml protein-A column at a flow rate of 0.5 ml/min. The column wasextensively washed with loading buffer at a flow rate of 2 ml/min. Boundantibody was eluted at a flow rate of 0.5 ml/min with 0.1 M of citricacid (pH 3) and neutralized with 1 M Tris/HCl (pH 9). Protein-containingfractions were combined, dialyzed against 5 liters of PBS (16 h, 4° C.),sterile filtered and stored at 4° C. Purified IgG antibodies wereanalyzed by 12%/SDS polyacrylamide gel electrophoresis under reducingand non-reducing conditions and stained with GELCODE® Blue (Pierce,USA). For Western blot, soluble cell extracts were separated by 12%/SDSpolyacrylamide gel electrophoresis under reducing and non-reducingconditions and electro-transferred onto nitrocellulose membrane. Heavyand light IgG chains were detected with HRP-conjugated goat anti humanantibodies (Jackson ImmunoResearch Laboratories, USA). The membrane wasdeveloped using the RENAISSANCE® Western blot Chemiluminescence Reagent(NEN, USA) according to the supplier's instructions.

C. Construction of Plasmid pBAD33-NlpA-ZZ

Plasmid pBAD33-NlpA-ZZ for expression of NlpA-ZZ fusion proteins wasconstructed as follows: A DNA fragment carrying the leader peptide andfirst six amino acids of the mature NlpA gene in frame with a scFvsequence was obtained as a XbaI/HindIII fragment from plasmid pAPEx1(Harvey et al., 2004) and introduced into plasmid pBAD33 that waslinearized by the same enzymes. The resulting plasmid was namedpBAD33-NlpA-scFv. Next, the ZZ gene flanked by NcoI and NotI sites wasrecovered from plasmid pET22-NN-ZZ-PE38 and inserted as NcoI/NotIfragment into plasmid pBAD33-NlpA-scFv digested with the same enzymes togenerate the final pBAD33-NlpA-ZZ.

D. Flow Cytometry Analysis

Evaluation of the ability of NlpA-ZZ displaying spheroplasts to capturefull length IgG antibodies added to the supernatant was performed asfollowed: E. coli JUDE-1 transformed with the pBAD33-NlpA-ZZ plasmidwere inoculated in TB medium supplemented with 30 μg/ml chloramphenicoland 2% (w/v) glucose and grown at 30° C. When the cultures reached to anA₆₀₀ of 1.0, the cells were pelleted for 5 min at 4500 rpm andresuspended in TB medium supplemented with 30 μg/ml chloramphenicol and0.2% arabinose. The cells were grown for 4 more hours at 25° C. Followedinduction, spheroplasts were prepared by permeabilization of thecellular outer membrane. Cells equivalent to ˜1 ml of A₆₀₀=5 werepelleted for 5 min at 8000 rpm in an microfuge and resuspended in 350 μlof ice-cold solution of 0.75 M sucrose/0.1 M Tris-HCl, pH=8. Next, 700μl of ice-cold 1 mM EDTA were added gently followed by addition of 100μg/ml hen egg lysozyme. The suspension was incubated at RT for 20 min,followed by addition of 50 μl of 0.5 M MgCl₂ and incubation on ice foradditional 15 min. The suspension was pelleted by centrifugation for 10min at 10,000 rpm at 4° C. and the resulting spheroplasts wereresuspended in 1 ml of PBS. For labeling, cells equivalent to ˜1 ml ofA₆₀₀=1 were incubated with 50 nM of FITC-conjugated Human IgG antibodiesor 100 nM of bacterially expressed M18.1 Hum IgG antibodies followed byincubation with 50 mM FITC-conjugated PA. For studying the NlpA-ZZ-IgGcomplex stability, labeling was done 5 nM of FITC-conjugated Human IgGantibodies and the residual fluorescence was analyzed by FC starting attime point zero and up to 72 hours incubation of the cells at 4° C. inthe dark.

E. Whole Cell-ELISA

The number of NlpA-ZZ molecule presented on the inner-membrane of JUDE-1induced cells and of the equilibrium dissociation constant (KD) of thecomplex NlpA-ZZ-IgG was determined by whole cell-ELISA assays asfollows. NlpA-ZZ induced spheroplasts equivalent to ˜1 ml of A₆₀₀=0.1(108 cells) were incubated with a two-fold dilutions serial ofHRP-conjugated-Rabbit IgG antibodies, starting with a concentration of 5nM for 1 h at RT in PBS containing 1% BSA (1% BPBS). After washing threetimes with the same buffer detection of bound IgG antibodies wasperformed by addition of 0.5 ml of the chromogenic HRP substrate TMB(DAKO, USA) to each tube and color development was terminated with 0.25ml of 1 M H₂SO₄. Finally, the tubes were centrifuged for 10 min at 4000rpm and color intensity of supernatants was measured at 450 nm. Thebinding-affinity was estimated as the IgG concentration that generates50% of the maximal signal. Estimation of the number of NlpA-ZZ moleculepresented on the inner-membrane of E. coli JUDE-1 induced cells wasbased on the assumption that the binding stoichometry of NlpA-ZZ to IgGis 1:1.

Example 3 Evaluation of the APEx-ZZ-IgG System by SimultaneousExpression of the NlpA-ZZ and IgG in the Host Cells and Enrichment ofCells Expressing M18.1 Hum IgG over Cells Expressing 26.10 IgG

To evaluate the ability of NlpA-ZZ cells to capture IgG moleculesexpressed endogenously in the periplasm of the same host cell, E. coliharboring the pBAD33-NlpA-ZZ were introduced with pMAZ360-M18.1 Hum-IgGor pMAZ360-26.10-IgG and analyzed by FC for simultaneous expression andcapturing of the IgG. As shown (FIG. 6A) when labeled with PA-FITC onlycells that coexpressed the M18.1 IgG were fluorescent while cellsco-expressing the 26.10 IgG which binds to an unrelated antigen, werenot stained. Similarly, incubation with Digoxin-BODYPY™, resulted inlabeling only of cells 26.10 IgG bound whereas those expression theM18.1 IgG gave background fluorescence (FIG. 6B). The specificity of thetwo antibodies to the relevant antigen was further confirmed (FIG. 6C,D).

It was further shown that NlpA-ZZ cells expressing the M18.1 IgG couldbe enriched from a large excess of cells expressing the 26.10 IgG.Specifically, spheroplasts expressing NlpA-ZZ and also displaying theM18.1 Hum IgG were mixed with a 100,000 fold excess of spheroplastsdisplaying the 26.10 IgG. Spheroplasts displaying the M18.1 Hum antibodywere enriched by two rounds of sorting using a Cytomation MOFLO™instrument, following labeling with fluorescent M18.1 Hum antigen,PA-FITC. Cells showing high fluorescence were collected and immediatelyresorted. Following each round of sorting, the IgG DNA was rescued byPCR and ligated into pMAZ360-IgG plasmid. Fresh NlpA-ZZ E. coli cellstransformed with the resulting plasmids were grown, induced for IgGexpression and subjected to next round of sorting against PA-FITCantigen. As shown in (FIG. 7) after two round of sorting M18.1 Hum IgGdisplaying cells were enriched to approximately 100% of the population.Sequence analysis revealed that following the first round of sorting7/20 randomly clones were M18.1 Hum, while following the second round10/10 came out as M18.1 Hum revealing complete enrichment of the desiredpopulation.

Example 4 Presentation of Expressed Antibodies

A. Flow Cytometry Analysis

The capture of IgG expressed in the periplasm by NlpA-ZZ expressedwithin the same cell was evaluated as followed: E. coli cells expressingNlpA-ZZ were co-transformed with plasmids pMAZ360-M18.1 Hum-IgG or withpMAZ360-26.10-IgG and single colonies were used to inoculate TB mediumsupplemented with 30 μg/ml chloramphenicol, 100 μg/ml ampicilin and 2%(w/v) glucose. Cultures were grown at 30° C. and when the opticaldensity reached A₆₀₀=1.0, the cells were pelleted for 5 min at 4,500rpm. The cells were resuspended in TB medium supplemented with 30 μg/mlchloramphenicol, 100 μg/ml ampicilin and isopropyl β-D-thiogalactoside(IPTG) to induce protein synthesis and growth was continued overnight at25° C. The following day cells were induced with 0.2% arabinose foradditional 3 hours at 25° C. for NlpA-ZZ expression. Followinginduction, spheroplasts were prepared as in example 2 and labeled with50 nM of the appropriate fluorescent probe at RT for 45 min beforeevaluation by FACS. To confirm the specificity of the bacteriallyexpressed IgGs, spheroplasts were incubated with 10 fold molar excess ofnon-cognizant antigen for 1 h at RT prior to incubation with thefluorescently labeled antigen.

B. Enrichment by FACS Sorting

Enrichment of NlpA-ZZ cells expressing the M18.1 Hum IgG from a 100,000fold excess of cells expressing the 26.10 IgG was performed as follows:Following preparation of spheroplasts displaying different IgGantibodies, as described in example 4. A, 10⁹ 26.10 IgG displaying cellswere mixed with 10⁴ cells displaying the M18.1 Hum IgG. The mixedculture was incubated with 100 nM of PA-FITC antigen for 45 min at RTand analyzed on a MOFLO™ droplet deflection flow cytometer by using a488-nm Argon laser for excitation. Cells were selected based onincreased fluorescence in the fluorescein emission wavelength using a530/40 band-pass filter. A total of 1.5×10⁸ cells were sorted at 25,000s⁻¹ and 2.25×10⁶ events (1.5%) were recovered. The collected events wereimmediately resorted through the flow cytometer at 250 s⁻¹ and 5×10⁴(6.5%) events were recovered. Subsequently, a DNA fragment correspondingto the VL-Ck-VH sequence of the IgG gene was amplified by PCR out of thesorted population using primers 5′ VL amplifier-FOR and 3′ VHamplifier-REV (Table 2). Once amplified, the resulting PCR products werethen recloned into pMAZ360-IgG vector, retransformed into NlpA-ZZ cells,and grown overnight on agar plates at 30° C. The resulting clones weregrown, induced for expression of IgG and subjected to a second round ofsorting during which a population of 8×10⁷ spheroplasts were sorted at arate of at 25,000 s⁻¹ and 2×10⁵ events (0.25%) were captured. Recoveredspheroplasts were immediately resorted to recover a final of 5×10³events (17.5%) at 200 s⁻¹.

Example 5 Construction and Screening of an Immunized Anti-PA IgG Library

Mice were immunized with the PA antigen (PA-83). After boosterimmunization the antibody titer elevated antibody titer was detected.Total RNA was recovered and cDNA was prepared using standard methods(REF), heavy and light chain variable domains were amplified using amodification of a primer set designed for the construction of mouse scFvlibraries (Krebber et al., 1997). The primer set was designed tofacilitate efficient cloning of the VH and VL variable domainsseparately as NheI/HindIII and NcoI/NotI restriction fragments,respectively, into the pMAZ360-IgG expression vector.

The anti-PA IgG library was constructed by two consecutive steps. First,the variable VH domains were ligated into the pMAZ360-IgG plasmid togenerate a library of 10⁷ independent transformants. The VH library wasevaluated for expression of full-length IgGs in E. coli periplasm byanalyzing 10 library clones selected at random. Western-blot analysisshowed that 70% of the randomly selected clones expressed intact IgGs(FIG. 8A). DNA analysis of 10 individual clones revealed that eachencoded a different VH sequence, as expected. DNA from the library waspooled and the VL variable domains were cloned into the VH libraryvector to generate a final anti-PA IgG library of 10⁷ independentclones. The accomplished IgG library was analyzed for expression of fullassembled IgG antibodies in the bacterial periplasm, showing 70%productivity for intact IgGs (FIG. 8B). DNA analysis of 10 randomlypicked clones confirmed sequence variation within the library. Theanti-PA IgG library was subjected to three rounds of sorting against PAantigen. Briefly, in each round library cells were grown underconditions facilitating induction and capturing of soluble IgG moleculesin the bacterial periplasm, cells were treated with Tris-EDTA-lysozyme,washed and labeled with 100 nM of PA-FITC for affinity and Alexa Flour647-Chicken anti-Human IgG for monitoring IgG expression. The librarywas subjected to three rounds of sorting using the MOFLO™ flow cytometerending with a significant enrichment signal as indicated by FC (FIG. 9).For screening analysis 200 clones from second round of sorting and 400clones from the third round of sorting were grown in 96-wells plates andinduced for expression of IgG, soluble cell extract of the individualclones was analyzed for direct binding to PA antigen via ELISA.Following sequence alignment of a comprehensive set of positive clones,5 individual clones were identified as unique (FIG. 10).

Example 6 Selection of Antibodies from an Expression Library

A. Construction of Immunized Anti-PA IgG Library

Construction of the anti-PA IgG library was performed as follow: Threefemale BALB/c mice of 6-8 weeks of age were immunized with the monomersubunit of Bacillus anthracis protective antigen (PA-83) (ListBiological Labs, USA) in complete Freunds adjuvant. Mice were boostedthree times at intervals of two weeks using 40 μg of PA-83 foe weeks 2and 4 and a final booster of 100 μg at week 6. The mice were bled threedays after each boost and the serum anti-PA antibody titer wasdetermined by an ELISA. Total RNA was extracted from 59 mg of spleentissue using MIRVANA™ miRNA Isolation Kit (AMBION®, USA). mRNA wasrecovered from total RNA using MICROPOLY(A)PURIST™ mRNA Purification Kit(AMBION®, USA) and cDNA was produced using SUPERSCRIPT™ III(INVITROGEN™, USA) with an anchored oligo-d(T) primer. Heavy and lightchain variable domains were amplified in PCR using a set of primers thatwas designed to facilitate efficient cloning of the VH and VL variabledomains separately into the pMAZ360-IgG expression vector. Constructionof the anti-PA IgG library was performed in two consecutive steps,initially; the variable VH domains were introduced into plasmidpMAZ360-IgG as NheI/HindIII restriction fragments to generate a libraryof 10⁷ independent VH domains. Following sequence validation of the VHlibrary the VL variable domains were cloned into the VH library vectoras NcoI/NotI to generate a final anti-PA IgG library of 10⁷ independentclones. Determination of full-length IgG expression in E. coli periplasmamong randomly selected clones was performed on a small scale of 1 mlessentially as described in example 2.B.

B. Library FACS Sorting.

Sorting of the anti-PA IgG library by FACS was performed as follow:Following preparation of spheroplast displaying IgGs as described inexample 4.A, cells equivalent to ˜1 ml of A₆₀₀=1 (10⁹ cells) wereincubated with 100 nM of PA-FITC antigen and Alexa Flour 647-Chickenanti-Human IgG Fc-specific (diluted 1:50) for 45 min at RT and analyzedon a MOFLO™ droplet deflection flow cytometer by using a 488-nm and a633 nm Argon lasers for excitation. Cells were selected based onimproved fluorescence in the fluorescein and Alexa Flour 647 emissionspectrum detecting through a 530/40 and a 670/20 band-pass filter,respectively. The library was subjected to three consecutive rounds ofsorting under the following conditions: in round one a total of 1.15×10⁸cells were sorted recovering 2.85×10⁶ events (2.5%) at 25,000 s⁻¹. Next,5.5×10⁵ of the spheroplasts captured were immediately resorted throughthe flow cytometer recovering 1.35×10⁵ events (22%) at 200 s⁻¹. Theconditions for round two and three were as follow: sort 2;6.45×10⁷/3.2×10⁵ (0.5%) at 25,000 s⁻¹; resort 2 3.55×10⁴/2.3×10⁴ (65%)at 250 s⁻¹; sort 3; 6.5×10⁷/3.4×10⁵ (0.5%) at 25,000 s⁻¹; resort 32.25×10³/1.65×10³ (72%) at 600 s⁻¹. Following each round of sorting aDNA fragment corresponding to the VL-Ck-VH sequence of the IgG gene wasamplified out of the sorted population by PCR using primers 5′ VLamplifier-FOR and 3′ VH amplifier-REV (Table 2). Once amplified, theresulting PCR products were recloned into pMAZ360-IgG vector,retransformed into NlpA-ZZ cells, and grown overnight on agar plates at30° C. The resulting clones were grown, induced for expression of IgGand subjected to additional round of sorting. Following three rounds ofsorting the PCR recovered IgG genes were ligated into pMAZ360-IgGexpression vector and transformed to fresh E. coli JUDE1 cells forexpression of soluble non-captured IgG antibodies.

C. Library Screening by Direct Binding ELISA.

Randomly selected single colonies from B above were used to inoculate200 μl of LB medium supplemented with 100 μg/ml ampicillin and 2% (w/v)glucose per well in U-bottom 96-well plates. Following over night growthat 30° C. with shaking at 150 rpm, the cutlers were subcultured bydilution of 1:20 to inoculate 200 μl of TB medium supplemented with 100μg/ml ampicillin and 2% (w/v) glucose in fresh 96-well plates and grownfor 3 hour at 30° C. with shaking at 150 rpm. Next, the plates werecentrifuged for 15 min at 4450 rpm and pellets were resuspended in TBmedium supplemented with 100 μg/ml ampicillin and 1 mMisopropyl-1-thio-D-galactopyranoside (IPTG). Cells were induced ON at25° C. with shaking at 150 rpm for expression of the selected IgGantibodies. The following day the plates were centrifuged and the cellwere lysed in 200 μl of 20% BUGBUSTER™ HT Protein Extraction Reagent inPBS for 20 min at RT. The plates were centrifuged as above and ofsoluble cell lysates were tested for direct binding to the PA antigen asfollows; ELISA plates were coated with 10 μg/ml of PA in PBS at 4° C.for 20 h and blocked with 2% (v/v) non-fat milk in PBS (MPBS) for 2 h atRT. Next, 25 μl of soluble cell lysates from library clones were appliedinto plates containing 75 μl of MPBS and incubated for 1 hour.Subsequently the plates were washed 3 times with PBST. Bound IgG wasdetected with HRP-conjugated goat anti human antibodies (×10,000dilution in MPBS). The ELISA was developed using the chromogenic HRPsubstrate TMB and color development was terminated with 1 M H₂SO₄. Theplates were read at 450 nm.

Example 7 Binding Characterization of Selected IgG Clones

Five clones that gave the highest ELISA signal were characterizedfurther. First, the 5 selected anti-PA IgG clones were tested forbinding to PA antigen by FACS (FIG. 11). The FACS signal exhibited bythe various clones is a function of the expression level and theaffinity for the PA antigen. To evaluate the expression levels and todetermine the binding affinities, the 5 selected IgG antibodies wereover expressed and purified on a protein A column. 12%/SDS-PAGE undernon-reducing conditions (FIG. 12A) and reducing conditions (FIG. 12B)revealed that the expression yields of purified material were between0.2-1 mg/l of cells.

Antigen association and dissociation rate constants and equilibriumdissociation constants were determined by Surface Plasmon Resonanceanalysis on a BIACORE® 3000 (BIACORE®) instrument using two differentmethods. The first is based on direct binding to antigen-immobilizedchip, performed essentially as described in (Harvey et al., 2004).Briefly, 750 response units of PA-83 or PA-63 were coupled to a CM5 chipin the same way described above. BSA was similarly coupled and used forin-line subtraction and the second involved IgG capturing by anti-Fcantibody followed by injection of the antigen as analyte. The secondmethod is a modification of a method described by (Canziani et al.,2004). Briefly, goat anti human IgG1 Fcγ fragment specific (JacksonLaboratories) was coupled to two in-line flow cells of a CM5 chip at 30μl/min to an equivalent of 10,000 response unit by using1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimidechemistry. Varying concentration of purified antibodies inHepes-buffered saline-EP buffer (BIACORE®) were injected at a flow rateof 5 μl/min at 25° C. to achieve approximately 500 response units ofcaptured IgGs. Buffer and antigen were then injected serially throughin-line flow cells at a flow rate of 50 μl/min (5 minute stabilization,1 minute association, 5 minute dissociation) and the surface wasregenerated using two pulses of 100 mM H₃PO₄. Three-fold dilution seriesof PA-63 starting at 15 nM were analyzed in duplicate using theBIAevaluation software (BIACORE®) with appropriate subtraction methods.All kinetic analysis were performed at 25° C. in Hepes-bufferedsaline-EP buffer at a flow rate of 100 ul/min and regeneration wasperformed with one injection of 4M MgCl₂. IgGs were injected at 10 nMand analyzed using BIAevaluation software.

Example 8 Screening of IgG Libraries by Phage Display

For IgG phage-display, E. coli K91K/F′ cells (Smith and Scott, 1993)expressing soluble IgG antibodies in the periplasm were infected withFUSE5-ZZ phage (Yacoby et al., 2006). These phage particles allowpolyvalent display of the IgG-Fc binding protein ZZ, on all copies ofthe p3 minor coat protein of filamentous bacteriophage. The FUSE5-ZZphage in this system performs not only as helper phage but also in turncapture the IgG antibody in the periplasm via the ZZ moiety. RescuedFUSE5-ZZ-IgG phage particles (which preferentially harbor the high copynumber pMAZ360-IgG phagemid over the replication-defective FUSE5-ZZgenome) are selected for antigen binding by several rounds of phagepanning (FIG. 14). FUSE5-ZZ phage particles although displaying the ZZprotein on all copies of the p3 minor coat protein are well capable toinfect and propagate in E. coli/F′ cells.

For the evaluation of the IgG phage-display system the anti-PA M18.1 HumIgG and the anti-digoxin 26.10 IgG served as models antibodies.Initially, purified FUSE5-ZZ phage particles were evaluated for theircapabilities to capture IgG in solution. Phage particles were preparedand mixed with purified M18.1 Hum IgG or 26.10 IgG antibodies. Afterwashing to remove any excess, unbound antibodies the phage particleswere analyzed by ELISA. Phage that had captured the anti-PA andantidigoxin antibodies showed a strong signal on plates coated with therespective antigens but not on plates coated with unrelated antigens.

FUSE5-ZZ phage formed in cells that also express IgG antibodies withinthe periplasm were shown to be capable of capture of the antibodies. E.coli K91K/F′ cells transformed with the phagemids pMAZ360-M18.1 Hum-IgGand with pMAZ360-26.10-IgG were grown under conditions permissive forphage infection. Following infection with FUSE5-ZZ phage, the cultureswere allowed to grow over night under conditions favorable for phageproduction. The following day phage particles were precipitated andevaluated for specific binding by direct ELISA (FIG. 15). The resultsobtained confirmed that assembled FUSE5-ZZ produced in cells expressingthe M18.1 Hum-IgG bound specifically to PA while phage produced in cellsexpressing the 26.10-IgG antibody bound specifically to plates coatedwith digoxin.

The IgG captured by the ZZ-displaying phage is kinetically very stableas competition of the bound IgGs by excess of unrelated IgG resulted inonly a low reduction in the specific ELSA signal. The stable interactionof the complex ZZ-IgG although non-covalent is most likely attributed tothe polyvalent display of the ZZ domain on all copies of the phage p3coat protein. Hence, the adjacent ZZ molecules contribute by aviditymanners to capture and retain the IgG molecules in a very stable way.

Example 9 Expression of Antibodies on Phage

A. Propagation of FUSE5-ZZ Phage

E. coli K91K/F′ were grown at 37° C. and harvested in exponential(logarithmic) growth phase (0.6≦A₆₀₀ nm≦0.8) to maximize the expressionof the F pili proteins. Cells transformed with FUSE5-ZZ DNA wereinoculated ON at 30° C. in 5 ml of 2×YT medium supplemented with 20μg/ml tetracycline and 50 μg/ml kanamycin. The following day cultureswere diluted into 500 ml of 2×YT medium supplemented with 20 μg/mltetracycline and grown overnight with shaking (250 rpm) at 30° C. Cellswere pelleted at 6000 rpm for 15 min at 4° C. and the supernatant wasfiltered and mixed with ⅕ volume of PEG/NaCl and incubated for 2 hr ormore on ice. Phage-precipitates were collected by centrifugation at7,800 rpm for 30 min at 4 C and the supernatant was carefully aspiratedoff. The phages were resuspended in sterile and filtered PBS and kept at4□C. To titer the phage stock, 10-fold serial dilutions of the phageswere made in sterile 2×YT medium. A logarithmic E. coli K91K/F′ culturewas infected with the diluted phages and the mixed culture was incubatedfor 30 min at 37° C. without shaking followed by incubation for 30 minat 37° C. with gentle shaking. Infected cells were plated on 2×YT platessupplemented with 20 μg/ml tetracycline and 50 μg/ml kanamycin and grownovernight at 37° C.

B. In Vitro Coupling of Purified IgGs to FUSE5-ZZ Phage

A stock solution of 5×10¹¹ CFU/ml FUSE5-ZZ phage particles was mixedwith 200 nM of purified M18.1 Hum IgG or 26.10 IgG in PBS. The mixturewas rotated head-over-head for two hours at RT. Next, phage particleswere precipitated with PEG-NaCl as described above; the supernatant wascarefully aspirated off and the phage were resuspended in sterile andfiltered PBS and kept at 4° C.

C. Preparation of FUSE5-ZZ-IgG Phage

For preparation of the FUSE5-ZZ-IgG phage particles, a single colony ofE. coli K91K/F′ cells transformed with phagemids pMAZ360-M18.1 Hum-IgGor with pMAZ360-26.10-IgG was grown ON at 30° C. in 5 ml of 2×YT mediumsupplemented with 100 μg/ml ampicillin, 50 μg/ml kanamycin and 2%glucose. The following day the culture was diluted 1:100 into 10 ml of2×YT medium supplemented with 100 μg/ml ampicillin and 2% glucose. Cellswere grown with shaking at 37° C. until 0.6≦A₆₀₀≦0.8 followed byaddition of FUSE5-ZZ helper phage at a ratio of 1:20 (number ofbacterial cells: helper phage particles, taking into account thatA₆₀₀=1˜10⁹ bacteria/ml). For infection, the cultures were incubatedwithout shaking at 37° C. for 60 min and then transferred to a shaking37° C. incubator at 110 RPM for 30 min. The infected cells werecollected by centrifugation and resuspended in 40 ml of 2×YT mediumsupplemented with 100 μg/ml ampicillin and 20 μg/ml. For phagesproliferation, the cultures were incubated with shaking (250 RPM) at 30°C. overnight. The following day, FUSE5-ZZ-IgG phage particles wereprecipitated with PEG-NaCl as described above; the supernatant wascarefully aspirated off and the phage were resuspended in sterile andfiltered PBS and kept at 4° C.

D. Phage-ELISA

ELISA plates were coated with 10 μg/ml of PA or 10 μg/ml of digoxin-BSAin PBS at 4° C. for 20 h and blocked with 2% (v/v) non-fat milk in PBS(MPBS) for 2 h at RT. Next, 50 μl of 10¹² CFU/ml of FUSE5-ZZ-IgG phageparticles were applied in a three-fold dilution series into platesalready containing 100 μl of MPBS and incubated for 1 hour at RT.

To evaluate the stability of binding of IgG onto the phage, 10¹⁰FUSE5-ZZ-IgG phage particles were incubated with 1 μM of commercialHuman IgG as a competitor for 1 h at RT and then the mixture was appliedonto ELISA plates. Following incubation the plates were washed 3 timeswith PBST. Bound phage particles were detected with HRP-conjugated goatanti M13 antibodies (×5,000 dilution in MPBS). The ELISA was developedusing the chromogenic HRP substrate TMB and color development wasterminated with 1 M H₂SO₄. The plates were read at 450 nm.

Example 10 Enrichment of FUSE5-ZZ-IgG Phage Particles by AntigenBio-Panning

10¹⁰ FUSE5-ZZ-M18.1 Hum-IgG phage particles were mixed with 10⁴FUSE5-ZZ-26.10-IgG and vice versa. The two mixed phage populations weresubjected to three rounds of bio-panning each on immunotubes coated withPA or digoxin-BSA, respectively. ELISA analysis of the isolated closerevealed a significant enrichment of the respective phages (FIG. 16A,FIG. 16B). Thus, intact IgG molecules can be efficiently expressed in E.coli periplasm and subsequently be captured and displayed on phagearticles that in turn can be screened for specific antigen enrichment.These experiments illustrate that IgG with a desired specificity can beselected directly from libraries.

Example 11 Selection of Antibody Presenting Phage

A. Phage Bio-Pannin.

A 35 mm tissue-culture six-well plate was coated overnight at 4° C. with75 μg/ml of PA or 100 μg/ml of digoxin-BSA in PBS in a total volume 0.7ml. After discarding the excess solution, the wells were washed oncewith PBS and blocked with 3 ml of PBS containing 0.25% (w/v) gelatin(PBSG) for two hours at RT. Next, the wells were washed five times withPBS and incubated with the suitable phage population in a total volumeof 0.7 ml on PBS and rocked gently at RT for two hours. Unbound phageswere rinsed away and the plate was washed extensively X10 with PBScontaining 0.05% (v/v) Tween 20 (PBST) followed by X5 washes with PBS.Bound phages were eluted with the addition of 400 μl of elution buffer(0.1 M HCl adjusted to pH 2.2 with glycine, 1 mg/ml of BSA) for tenminutes at room temperature with gentle agitation. The eluted phageparticles were transferred into a 1.5 ml microfuge tube and neutralizedwith 75 μl of neutralizing buffer (1 M Tris-HCl, pH 9). The selectedphages were used for re-infection of E. coli K91K/F′ cells forenrichment. 0.5 ml of the neutralized eluted phages were mixed with 4.5ml 2×YT medium and 5 ml of logarithmic K91K/F′ cells. For infection,cultures were incubated without shaking at 37° C. for 60 min and thentransferred to a shaking 37° C. incubator at 110 RPM for 30 min. Theinfected cells were spread on 2×YT plates supplemented with 100 μg/mlampicillin, 50 μg/ml kanamycin and 2% glucose and grown overnight at 30°C. Titration of output and input phages was done after every panningcycle. Phages at 10-fold serial dilutions were used to infectlogarithmic E. coli K91K/F′ cells. The cultures were incubated for 30-60min at 37° C. 10 ml samples were plated in duplicates on 2×YT platessupplemented with 100 μg/ml ampicillin, 50 μl/ml kanamycin and 2%glucose and grown overnight at 30° C.

Preparation of input phage was performed as follows: Followingincubation of the plates ON at 30° C., the cells were scraped andsub-cultured into 50 ml of 2×YT medium supplemented with 100 μg/mlampicillin, 50 μg/ml kanamycin and 2% glucose to give a startingA600=0.1. The cultures were grown with shaking at 37° C. until0.6≦A600≦0.8 followed be removal of 10 ml of each culture for infectionwith FUSE5-ZZ helper phage. Infection and production of FUSE5-ZZ-IgGphage particles was performed essentially as described in example 10.C.Evaluation of the enrichment of the two mixed population was performedby ELISA as described in example 10.D.

Example 12 Construction of a Semi-Synthetic Chimeric IgG Library Basedon a Single-Framework Scaffold

A very large semi-synthetic IgG library with a total size of 3×10⁹clones was constructed. The library was designed using the scaffold ofthe anti-PA YMF10 IgG that was isolated from the immunized anti-PAlibrary. Clone YMF10 variable V_(H) and Vκ genes were selected asscaffold donors since the resultant full-length of this antibodydemonstrated high expression levels in E. coli periplasm and efficientassembly of the heavy and light chain into full-length IgG. Further,this antibody belongs to the canonical family 12211 which is the mostcommon among antibodies and is multi-specific in its bindingcapabilities toward a variety of antigens, including binders toproteins, surface antigens, nucleic acids and small molecules such ashaptens (Vargas-Madrazo et al., 1995).

For single-framework libraries, diversity depends solely on thedegeneracy of synthetic DNA used to vary the CDR loop sequences.Randomization was introduced to both CDR3 regions of YMF10 V_(H) and Vκgenes simultaneously, since these two regions form the inner-circle ofthe antigen binding site, and therefore show the highest frequency ofantigen contacts in structurally known antibody-antigen complexes. Inthis example the DNA sequence of clone YMF10 variable genes was alignedwith all functional sequences of antibodies deposited in the IgBLASTdata base (on the World Wide Web at ncbi.nlm.nih.gov/igblast/) and theCDR3 sequences of approximately 500 clones that scored the highesthomology were aligned and at each position the amino acids were rankedby the frequency of occurrence in CDRH3 (Table 3) and CDRL3 (Table 4).The rational behind this approach is that within the CDRs, particularresidue positions are not as hypervariable as classified. For example,in naturally occurring CDRs a propensity towards the occurrence of Tyr,Ser, Trp, Gly and Asn residues is notable (de Kruif et al., 1995;Fellouse et al., 2007). These residues possess structural and functionalcharacteristics that appear highly desirable for antibody binding sites.The inventors reasoned that such “designed” CDR3 regions would increasethe frequency of cells expressing functional antibodies, thus resultingin a more efficient exploitation of library size which is limited tobacterial transformation capabilities.

In the Kabat numbering scheme, CDRH3 is defined as the sequence frompositions 95-102. To obtain the highest degree of diversity in CDR3 ofV_(H), six degenerate oligos were designed encoding 7-12 residues (Table3), corresponding to the most frequent distribution of CDRH3 length infunctional antibodies. Randomization of CDRL3 was carried out using onedegenerate oligo introducing only limited randomization to selectedpositions while other residues that are considered as consensus amongantibodies were kept as in the parental clone (Table 4). At theN-terminal boundary of CDRH3, additional randomization was introduced tothe parental sequence at positions 93-94 of the framework correspondingto the sequence found most commonly amongst natural antibodies. Thus,for both the heavy and light chain CDR3 regions, diversity was designedin a way that at most amino acid positions, the most frequent residuesin natural antibodies (bold amino acids in Tables 3 and 4) were encodedby a tailored degenerate codon (italicized nucleotides in Tables 3 and4) while minimizing the residues not included in the target diversity(non-bold amino acids in Tables 3 and 4). The over all coverage of thetarget diversity for both Table 3 and 4 was in the 75-95% range.

The semi-synthetic IgG library was constructed in two consecutive stepsusing DNA of plasmid pMAZ360-YMF10-IgG as a template. First, thevariable Vκ genes were amplified in a PCR reaction using the set ofprimers listed in Table 5 (NotI and HindIII sites in CDRL3 and CDRH7-12primers, respectively, are underlined in Table 5). The resulting set ofvariable Vκ domains were introduced into plasmid pMAZ360-IgG asNcoI/NotI restriction fragments to generate a library of 10⁷ independenttransformants. Next, the variable V_(H) genes were amplified in sixindependent PCR reactions corresponding for the six different length ofCDRH3 that varied between 7-12 amino acids using the set of primerslisted in (Table 5). The resulting set of variable V_(H) genes wasinserted separately into the Vκ library vector as NheI/HindIII fragmentsgenerating six separate libraries in the range of 4-6×10⁸ independenttransformants. Incorporation of all six libraries together gave rise toa final library with a total size of 3×10⁹ IgG clones. All PCRs werecarried out in a volume of 50 μl with 200 μM dNTPs, 1 μM of each primerand 2 units of Vent® DNA Polymerase (NEB, MA). PCRs consisted of 35cycles of 30 sec at 94° C., 1 min at 58° C. and 1 min at 72° C.

Determination of full-length IgG expression in E. coli periplasm amongrandomly selected clones was performed on a small scale of 1 mlessentially as described in Example 2. B. and the yield was determinedby western-blot analysis. Approximately 80% of the clones expressedfull-length antibodies (FIG. 17). DNA analysis of clones carryinginserts of full-length IgGs confirmed sequence variation within CDRH3and CDRL3 of library members. At most positions, the highest-rankingamino acid residues were encoded by the tailored degenerate codons withan overall coverage of the target diversity in the 75-95% range.

Example 13 Screening of the Semi-Synthetic IgG Library

Library Two-Color FACS Sorting

To demonstrate the capabilities of the semi-synthetic library to produceantibodies against an array of antigens, the human Annexin V, a 35 kDaphospholipids binding protein was selected as a target antigen. Sortingof the semi-synthetic library was performed using a two-color FACS asfollow: in each round library cells were grown under conditionsfacilitating induction and capturing of soluble IgG molecules in thebacterial periplasm. Cells were treated with Tris-EDTA-lysozyme, washedand labeled with 500 nM of FITC-conjugated Human Annexin V for affinityand (1:50 dilution) of Alexa Flour 647-Chicken anti-Human IgG formonitoring IgG expression. To determine appropriate settings forsorting, control JUDE-1 cells expressing only NlpA-ZZ protein weredouble-labeled with 500 nM of FITC-conjugated Human Annexin V and AlexaFlour 647-Chicken anti-Human IgG and analyzed for fluorescent emissionsignals on an ARIA™ droplet deflection flow cytometer carrying a 488-nmand a 633 nm Argon lasers for excitation. The control population wasused to set the appropriate double-negative quadrant (FIG. 18A). Forlibrary sorting a sorting gate in the double-positive quadrantcorresponding to approximately 5% of the total population was set (FIG.18B). In round one a total of 1×10⁹ cells were selected based onimproved fluorescence in the fluorescein and Alexa Flour 647 emissionspectrum detecting through a 530/40 and a 670/20 band-pass filters,recovering 5×10⁷ events at 20,000 s⁻¹. The collected speheroplasts wereimmediately resorted on the flow cytometer using the same collectiongate for the initial sort, recovering 5×10⁶ events (10%) at 200 s⁻¹.Subsequently, a DNA fragment corresponding to the Vκ-Cκ-V_(H) sequenceof the IgG gene was rescued by PCR, recloned into pMAZ360-IgG vector andtransformed into fresh E. coli JUDE-1-NlpA-ZZ cells. The resultingclones were grown, induced for expression of IgG and subjected to fouradditional rounds of sorting under the following conditions: sort 2;1×10⁸/5×10⁷ (5%) at 20,000 s⁻¹; resort 21×10⁷/1×10⁶ (10%) at 200 s⁻¹;sort 3; 5×10⁷/1×10⁶ (2%) at 20,000 s⁻¹; resort 3 1×10⁶/5×10⁴ (5%) at 200s⁻¹; sort 4; 5×10⁷/1×10⁶ (2%) at 20,000 s⁻¹; resort 4 1×10⁶/5×10⁴ (5%)at 200 s⁻¹; sort 5; 5×10⁷/1×10⁶ (2%) at 20,000 s⁻¹; resort 5 1×10⁶/5×10⁴(5%) at 200 s⁻¹.

Following five rounds of FACS sorting, randomly single selected cloneswere grown in 96-well plates, induced for expression of soluble IgGantibodies and analyzed for direct binding to Human Annexin V antigen inELISA using the same conditions described in Example 6C. Sequencealignment of positive clones that confirmed the highest ELISA signalrevealed four unique sequences (Table 6).

Example 14 Characterization of Annexin V Antibodies

Soluble cell extract of the individual clones was analyzed for antigendissociation kinetics using Surface Plasmon Resonance (SPR) analysis bydirect binding to Human Annexin V-immobilized chip. Briefly, for smallscale production of soluble cell extracts, E. coli JUDE1 cellstransformed with pMAZ360-IgG expression vectors were inoculatedovernight (ON) at 30° C. in Luria broth (LB) medium supplemented with100 μg/ml ampicillin and 2% (w/v) glucose. The following day, thecultures were diluted into 20 ml of Terrific broth (TB) mediumsupplemented with 100 μg/ml ampicillin and 2% (w/v) glucose to give astarting A₆₀₀ of 0.2 and grown at 30° C. When the cultures reached anA₆₀₀ of 1.0, the cells were pelleted for 10 min at 6000 rpm andresuspended in TB medium supplemented with 100 μg/ml ampicillin and 1 mMisopropyl-1-thio-β-D-galactopyranoside (IPTG). Cells were induced ON at25° C. for expression of IgG. The following day cells were harvested andpelleted for 10 min at 6000 rpm. Cell extracts were prepared byresuspension of the pellet in 5 ml of Hepes-buffered saline-EP bufferfollowed by two cycles of sonication. The extracts were clarified bycentrifugation at 12,000 rpm at 4° C. Antigen dissociation rateconstants were determined on a BIACORE® 3000 (BIACORE®) instrumentessentially as described in Example 7. Varying dilutions of clarifiedcell extracts in HEPES-buffered saline-EP buffer were injected at a flowrate of 50 μl/min at 25° C. to achieve approximately 200 response unitsof captured IgGs. Samples were analyzed in duplicate using theBIAevaluation software (BIACORE®) with appropriate subtraction methods.The selected clones exhibited dissociation rates that varied between1−35×10⁻² sec⁻¹ (Table 6).

TABLE 2 List of PCR primers Oligonucleotide Sequence HindIII-Mut-FOR5′-GCATAGTGATATCGCGTGCTTGACCT GTGAAGTG-3′ (SEQ ID NO: 5) HindIII-Mut-REV5′-CACTTCACAGGTCAAGCACGCGATAT CACTATGC-3′ (SEQ ID NO: 6) CH1-FOR5′-TCCTCGGCAAGCTTCAAGGGCCCATC G-3′ (SEQ ID NO: 7) CH3-AscI-REV5′-CTATGCGGCGCGCCTTATTATTTACC CGGAGACAGGGGGAG-3′ (SEQ ID NO: 8)M18.1-VH-NheI-FOR 5′-GAAATCCCTATTGCCTACGGCAGCCGCTGGATTGTTATTGCTAGCGGCTCAGCCG GCAATGGCGGAGGTCCAACTGGTTGAATC TG-3′(SEQ ID NO: 9) M18.1-VH-HindIII-REV 5′-GAAGCTTGCCGAGGAGACGGTGACCA GGG-3′(SEQ ID NO: 10) CK-FOR 5′-CGTACCACTGCGGCCGCACCATCTGT C-3′(SEQ ID NO: 11) CK-Stop-REV 5′-GCTTCAACAGGGGAGAGTGCTAATAATATATATATATATATATTCTAGAGAAGGA GATATACATATGAAATCCCTATTGCC-3′(SEQ ID NO: 12) M18.1-VL-NcoI-FOR 5′-AGCCGGCCATGGCGGATATTCAGATGACACAGTCC-3′ (SEQ ID NO: 13) M18.1-VL-Not-REV5′-GGCCGCAGTGGTACGTTTTATTTCAA CCTTGGTG-3′ (SEQ ID NO: 14)26.10-VH-NheI-FOR 5′-GTTATTGCTAGCGGCTCAGCCGGCAATGGCGCAGGTGGAGCTGCAGCAGTCT G-3′ (SEQ ID NO: 15) 26.10-VH-HindIII-REV5′-CCCTTGAAGCTTGCAGAGCTCACAGT AACACTAGCACCATGACC-3′ (SEQ ID NO: 16)26.10-VL-SfiI-FOR 5′-ATCGCGGCCCAGCCGGCCATGGCGGA CATAGTACTGACCCAGTC-3′(SEQ ID NO: 17) 26.10-VL-Not-REV 5′-GATGGTGCGGCCGCAGTGGCCGATTTGATCTCGAGC-3′ (SEQ ID NO: 18) 5′VL amplifier-FOR5′-CGGATAACAATTTCACACAGG-3′ (SEQ ID NO: 28) 3′VH amplifier-REV5′-AGTTCCACGACACCGTCACCG-3′ (SEQ ID NO: 29)

TABLE 3 Diversity designs for randomizing heavy chain CDRH3 Degen- YMF10N93 A94 G95 T96 P97 F98 E99 G100 L100a R100b R100c A100d D101 Y102Length eracy Size Oligo- CDRH3- RMC SSA BNK NNC NVC NWC DRK KVC BHC BVCBVC WTK GMC TAC 12   8 × 10⁹ 6 × 10⁸ nucleo- 12 tide CDRH3- RMC SSA BNKNNC NVC • DRK KVC BHC BVC BVC WTK GMC TAC 11   1 × 10⁹ 4 × 10⁸ 11 CDRH3-RMC SSA BNK NNC NVC • • KVC BHC BVC BVC WTK GMC TAC 10   9 × 10⁷ 6 × 10⁸10 CDRH3- RMC SSA BNK NNC NVC • • • BHC BVC BVC WTK GMC TAC 9 1.5 × 10⁷5 × 10⁸ 9 CDRH3- RMC SSA BNK NNC NVC • • • • BVC BVC WTK GMC TAC 8 1.7 ×10⁶ 5 × 10⁸ 8 CDRH3- RMC SSA BNK NNC NVC • • • • • BVC WTK GMC TAC 71.9 × 10⁵ 4 × 10⁸ 7 Amino A R A T G F E A L A A M A Y acids T P R G P YD S Y R R F D N A D S S L Y D S G G L D G G Y T D G G A S S I F A Y N RY H Y Y S L D V S C P P P W R A I W F D D Y P R H T D C C V N N K V H HP D C N T C H C L H H I E F Q C

TABLE 4 Diversity designs for randomizing light chain CDRL3 YMF10 Q89Q90 Y91 S92 S93 Y94 P95 L96 T97 Length Degeneracy Size OligonucleotideCDRL3 CAG MAK NRC DMC MRC HMC CCT YWC ACG 9 1.85 × 10⁴ 1 × 10⁷Amino acids Q Q Y N R N P L T H H D N S F N S S H T Y K G T S Y H N Y PD A H R C

TABLE 5 Oligonucleotide primers used to create semi-synthetic IgG library CDRL3 GATGGTGCGGCCGCAGTACGTTTAATCTCCAGCTTGGTCCCAGCACCGAACGTGWRAGGGKDGYKGKHGYNMTKCTGACAGAAA TAATCTGCCAAGTCTTCAGACTGC(SEQ ID NO: 30) VL-FOR CGGATAACAATTTCACACAGG (SEQ ID NO: 28) CDRH3-12CCCTTGAAGCTTGCTGAGGAGACTGTGAGAGTGGTGCCTTGGCCCCAGTAGKCMAWGBVGBVGDVGBMMYHGWNGBNGNNMNVT SSGKYACAGTAATAGACGGCAGTGTCCTC(SEQ ID NO: 31) CDRH3-11 CCCTTGAAGCTTGCTGAGGAGACTGTGAGAGTGGTGCCTTGGCCCCAGTAGKCMAWGBVGBVGDVGBMMYHGBNGNNMNVTSSG KYACAGTAATAGACGGCAGTGTCCTC(SEQ ID NO: 32) CDRH3-10 CCCTTGAAGCTTGCTGAGGAGACTGTGAGAGTGGTGCCTTGGCCCCAGTAGKCMAWGBVGBVGDVGBMGBNGNNMNVTSSGKYA CAGTAATAGACGGCAGTGTCCTC(SEQ ID NO: 33) CDRH3-9 CCCTTGAAGCTTGCTGAGGAGACTGTGAGAGTGGTGCCTTGGCCCCAGTAGKCMAWGBVGBVGDVGBNGNNMNVTSSGKYACAG TAATAGACGGCAGTGTCCTC(SEQ ID NO: 34) CDRH3-8 CCCTTGAAGCTTGCTGAGGAGACTGTGAGAGTGGTGCCTTGGCCCCAGTAGKCMAWGBVGBVGBNGNNMNVTSSGKYACAGTAA TAGACGGCAGTGTCCTC(SEQ ID NO: 35) CDRH3-7 CCCTTGAAGCTTGCTGAGGAGACTGTGAGAGTGGTGCCTTGGCCCCAGTAGKCMAWGBVGBNGNNMNVTSSGKYACAGTAATAG ACGGCAGTGTCCTC(SEQ ID NO: 36) VH-FOR TGGATAACGCCCTCCAATCGG (SEQ ID NO: 37)

TABLE 6 Dissociation kinetics and amino acid sequencesof CDR3s of isolated IgG clones Antibody K_(dis) CDR3 clone (sec⁻¹)V_(κ) V_(H) Parental 1.52 × 10⁻² QQYSSYPLT (SEQ ID NO: 38) GTPFEGLRRADY(SEQ ID NO: 39) YMF10 E5-3 1.04 × 10⁻² QNHDHHPHT (SEQ ID NO: 40)LPPDFRAIAY (SEQ ID NO: 41) D4-2 2.04 × 10⁻² QNHTSPSHT (SEQ ID NO: 42)VLYDSSGLAY (SEQ ID NO: 43) F9-2 1.52 × 10⁻¹ QNHTHPPLT (SEQ ID NO: 44)LYSCYYPFAY (SEQ ID NO: 45) A8-2 3.52 × 10⁻¹ QQGYHSPHT (SEQ ID NO: 46)VRLHDFCSLAY (SEQ ID NO: 47)

All of the methods disclosed and claimed herein can be made and executedwithout undue experimentation in light of the present disclosure. Whilethe compositions and methods of this invention have been described interms of preferred embodiments, it will be apparent to those of skill inthe art that variations may be applied to the methods and in the stepsor in the sequence of steps of the method described herein withoutdeparting from the concept, spirit and scope of the invention. Morespecifically, it will be apparent that certain agents which are bothchemically and physiologically related may be substituted for the agentsdescribed herein while the same or similar results would be achieved.All such similar substitutes and modifications apparent to those skilledin the art are deemed to be within the spirit, scope and concept of theinvention as defined by the appended claims.

REFERENCES

The following references, to the extent that they provide exemplaryprocedural or other details supplementary to those set forth herein, arespecifically incorporated herein by reference.

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1. A Gram negative bacterial cell wherein the cell comprises: a) anantibody-binding domain in the periplasm of the Gram negative bacterialcell; and b) an antibody bound to said antibody-binding domain; whereinsaid bacterial cell comprises different nucleic acid sequences encodingboth i) the antibody-binding domain and ii) the antibody, and whereinthe antibody comprises an Fc domain.
 2. The Gram negative bacterial cellof claim 1, wherein the bacterial cell is an E. coli bacterial cell. 3.The Gram negative bacterial cell of claim 1, wherein the antibody is afull length antibody.
 4. The Gram negative bacterial cell of claim 1,wherein the antibody is an IgG antibody.
 5. The Gram negative bacterialcell of claim 1, further defined as a population of bacterial cells saidpopulation collectively comprising a plurality of distinct antibodies.6. The Gram negative bacterial cell of claim 1, wherein the bacterialcell lacks an outer membrane.
 7. The Gram negative bacterial cell ofclaim 1, wherein an antibody-binding domain is an Fc- gamma RII,Fc-gamma RIII, FeRn, MRP protein, Protein B, protein A, protein G,protein H, Protein sbi, Allergen Asp fl 1, Allergen Asp fl 2 or AllergenAsp fl
 3. 8. The Gram negative bacterial cell of claim 1, wherein theantibody binding domain is anchored to the inner membrane of the Gramnegative bacterial cell.
 9. The Gram negative bacterial cell of claim 8,wherein the antibody-binding domain comprises a membrane anchoringpolypeptide fused to the N or C terminus of the antibody-binding domain.10. The Gram negative bacterial cell of claim 9, wherein the membraneanchoring polypeptide is the first six amino acids encoded by the E coliN1pA gene, one or more of transmembrane α-helices from an E coli innermembrane protein, or an inner membrane lipoprotein.
 11. The Gramnegative bacterial cell of claim 1, wherein the antibody-binding domainbinds to the constant region (Fc) of an antibody.
 12. The Gram negativebacterial cell of claim 11, wherein the antibody-binding domaincomprises a mammalian, bacterial or synthetic Fe binding domain.
 13. TheGram negative bacterial cell of claim 12, wherein the bacterial Fcbinding domain is a S. aureus protein A or protein G domain.
 14. TheGram negative bacterial cell of claim 12, wherein the synthetic Fcbinding domain is a polypeptide comprising a ZZ antibody binding domainof S. aureus protein A (ZZ polypeptide).
 15. The Gram negative bacterialcell of claim 1, wherein the nucleic acid encoding said antibody furthercomprises an immunoglobulin variable domain heavy (H) chain.
 16. TheGram negative bacterial cell of claim 15, wherein the nucleic acidencoding said antibody comprises an immunoglobulin light chain (L) andheavy (H) chain.
 17. The Gram negative bacterial cell of claim 16,wherein the immunoglobulin light chain (L) and heavy (H) chain comprisea dicistronic expression cassette.
 18. The Gram negative bacterial cellof claim 16, wherein the immunoglobulin light chain (L) and heavy (H)chain comprise a monocistronic expression cassette.
 19. The Gramnegative bacterial cell of claim 1, further defined as a population ofbacterial cells said population collectively comprising a plurality ofdistinct antibodies and a plurality of nucleic acids encoding saidantibodies.
 20. A method of selecting a bacterial cell comprising anantibody having specific affinity for a target ligand comprising thesteps of: i) obtaining a population of Gram negative bacterial cellsaccording to claim 19; iii) contacting the bacterial cells with a targetligand under conditions wherein the target ligand contacts the antibody;and iv) selecting at least one bacterial cell based on binding of thedistinct antibody to target ligand.
 21. The method of claim 20, whereintarget ligand is immobilized.
 22. The method of claim 20, wherein theselecting is carried out by FACS or magnetic separation or by adsorptionof the cells onto a surface containing immobilized antigen.
 23. Themethod of claim 20, further comprising removing target ligand not boundto the antibody.
 24. The method of claim 20, wherein the target ligandis a cancer-associated protein, a cell surface protein, an enzyme, avirus, a glycoprotein or a cell receptor ligand.
 25. The method of claim20, wherein said population of Gram negative bacterial cells is producedby a method comprising the steps of: (a) preparing a plurality ofnucleic acid sequences encoding a plurality of distinct antibodies; and(b) transforming a population of Gram negative bacteria with saidnucleic acid sequences wherein the Gram negative bacterial cellscomprise an antibody-binding domain anchored to bacterial innermembrane.
 26. The method of claim 20, wherein the plurality of nucleicacids encoding said antibodies comprise known sequences flanking thesequence encoding said antibody.
 27. The method of claim 20, whereintarget ligand is labeled.
 28. The method of claim 27, wherein targetligand is labeled with a fluorophore, a radioisotope or an enzyme. 29.The method of claim 20, further comprising (ii) disrupting the outermembrane of the bacterial cells before contacting the bacterial cellswith a target ligand.
 30. The method of claim 29, wherein disrupting theouter membrane of the bacterial cell comprises heating the bacterialcell with a combination of physical, chemical and enzyme disruption ofthe outer membrane.
 31. The method of claim 29, wherein disrupting theouter membrane of the bacterial cell further comprises removing theouter membrane of said bacterium.
 32. The method of claim 29, whereindisrupting the outer membrane of the bacterial cell comprises treatmentwith hyperosmotic conditions, treatment with physical stress, infectingthe bacterium with a phage, treatment with lysozyme, treatment withEDTA, treatment with a digestive enzyme or treatment with a chemicalthat disrupts the outer membrane.
 33. The method of claim 32, whereindisrupting the outer membrane of the bacterial cell comprises acombination of said methods.
 34. The method of claim 20 , wherein theselecting of step (iv) is further defined as comprising at least tworounds of selection wherein the sub-population of bacterial cellsobtained in the first round of selection is subjected to at least asecond round of selection based on the binding of the distinct antibodyto target ligand.
 35. The method of claim 34, comprising two to tenrounds of selection.
 36. The method of claim 20, further comprisingcontacting the bacterial cells with at least two target ligands.
 37. Themethod of claim 36, wherein the at least two target ligands comprisedistinct labels.
 38. The method of claim 36, further comprisingselecting bacterial cells based on binding of the distinct antibody toat least two target ligands.
 39. The method of claim 36, furthercomprising selecting bacterial cells based on binding of the antibody toat least one target ligand and based on the antibody not binding to atleast one target ligand.
 40. The method of claim 20, wherein theplurality of nucleic acids encoding said antibodies each furthercomprises an immunoglobulin variable domain heavy (H) chain.
 41. Themethod of claim 40, wherein the plurality of nucleic acids encoding saidantibodies each comprise an immunoglobulin light chain (L) and heavy (H)chain.
 42. The method of claim 20, further defined as a method ofproducing a nucleic acid sequence encoding an antibody having a specificaffinity for a target ligand and further comprising the step of: v)cloning a nucleic acid sequence encoding the antibody from the bacterialcell to produce a nucleic acid sequence encoding an antibody having aspecific affinity for a target ligand.
 43. The method of claim 42,wherein cloning comprises amplification of the nucleic acid sequence.44. The method of claim 42, further defined as a method of producing anantibody having a specific affinity for a target ligand furthercomprising the step of: vi) expressing a nucleic acid sequence encodingthe antibody to produce an antibody having a specific affinity for atarget ligand.